doc: update contribute.html

Fixes #4582.

* mentioned hg upload.
* added section on hg file.
* added small mention about being inside $GOROOT.
* added hg revert @NNNN
* reorganise the hg submit section for the common case of a non committer.
* made the Copyright section h2
* added note about leaving copyright years unchanged.

R=golang-dev, metanata4, shivakumar.gn, minux.ma, adg, shanemhansen
CC=golang-dev, metanata4
https://golang.org/cl/7278047

doc/contribute.html

@@ -112,7 +112,9 @@ you are a committer (see below), but Mercurial complains if it is missing.
 
 <p>
 After adding the extension, <code>hg help codereview</code>
-will show documentation for its commands.
+will show documentation for its commands. As the codereview extension is only
+enabled for your checkout in <code>$GOROOT</code>, the remainder of this 
+document assumes you are inside <code>$GOROOT</code> when issuing commands.
 </p>
 
 <h3>Log in to the code review site.</h3>
@@ -282,22 +284,37 @@ which <code>hg change</code> will print, something like:
 CL created: http://codereview.appspot.com/99999
 </pre>
 
+<h3>Adding or removing files from an existing change</h3>
+
 <p>
 If you need to re-edit the change description, or change the files included in the CL,
-run <code>hg change 99999</code>. Alternatively, you can use <code>hg file 99999 somefile</code>
-to add <code>somefile</code> to CL 99999, and use <code>hg file -d 99999 somefile</code> to remove
-<code>somefile</code> from the CL.
+run <code>hg change 99999</code>.
 </p>
 
 <p>
-Creating the change uploads a copy of the diff to the code review server, but it does not
-notify anyone about it. To do that, you need to run <code>hg mail</code> (see below).
+Alternatively, you can use
+<p/>
+
+<pre>
+$ hg file 99999 somefile
+</pre>
+
+<p>
+to add <code>somefile</code> to CL 99999, and
 </p>
 
+<pre>
+$ hg file -d 99999 somefile
+</pre>
+
 <p>
-You can see a list of your pending changes by running <code>hg pending</code> (<code>hg p</code> for short).
+to remove <code>somefile</code> from the CL.
 </p>
 
+<p>
+A file may only belong to a single active CL at a time. <code>hg file</code>
+will issue a warning if a file is moved between changes.
+</p>
 
 <h3>Synchronize your client</h3>
 
@@ -390,6 +407,12 @@ changes, but you may still need to run
 
 <h3>Mail the change for review</h3>
 
+<p>
+Creating or uploading the change uploads a copy of the diff to the code review server,
+but it does not notify anyone about it. To do that, you need to run <code>hg mail</code>
+(see below).
+</p>
+
 <p>To send out a change for review, run <code>hg mail</code> using the change list number
 assigned during <code>hg change</code>:</p>
 
@@ -411,6 +434,10 @@ $ hg mail -r golang-dev@googlegroups.com --cc math-nuts@swtch.com 99999
 
 <p>Note that <code>-r</code> and <code>--cc</code> cannot be spelled <code>--r</code> or <code>-cc</code>.</p>
 
+<p>
+If your change relates to an open issue, please add a comment to the issue
+announcing your proposed fix, including a link to your CL.
+</p>
 
 <h3>Reviewing code</h3>
 
@@ -424,7 +451,18 @@ to send comments back.
 
 <h3>Revise and upload</h3>
 
-<p>You will probably revise your code in response to the reviewer comments.
+<p>
+You will probably revise your code in response to the reviewer comments. When
+you have done this, you can upload your change to the code review server
+without sending a notification by running <code>hg upload</code> using the change
+list number assigned during <code>hg change</code>
+</p>
+
+<pre>
+$ hg upload 99999
+</pre>
+
+<p>
 When you have revised the code and are ready for another round of review, run
 </p>
 
@@ -445,39 +483,58 @@ The reviewer approves the change by replying with a mail that says
 <code>LGTM</code>: looks good to me.
 </p>
 
-<h3>Submit the change after the review</h3>
+<p>
+You can see a list of your pending changes by running <code>hg pending</code> (<code>hg p</code> for short).
+</p>
+
+<h3>Reviewing code by others</h3>
 
 <p>
-After the code has been <code>LGTM</code>'ed, it is time to submit
-it to the Mercurial repository.
-If you are a committer, you can run:
+You can import a CL proposed by someone else into your local Mercurial client
+by using the <code>hg clpatch</code> command. Running
 </p>
 
 <pre>
-$ hg submit 99999
+$ hg clpatch 99999
 </pre>
 
 <p>
-This checks the change into the repository.
-The change description will include a link to the code review,
-and the code review will be updated with a link to the change
-in the repository.
+will apply the latest diff for CL 99999 to your working copy. If any of the
+files referenced in CL 99999 have local modifications, <code>clpatch</code>
+will refuse to apply the whole diff. Once applied, CL 99999 will show up in
+the output of <code>hg pending</code> and others.
 </p>
 
 <p>
-If your local copy of the repository is out of date,
-<code>hg submit</code>
-will refuse the change:
+To revert a CL you have applied locally, use the <code>hg revert</code>
+command. Running
 </p>
 
 <pre>
-$ hg submit 99999
-local repository out of date; must sync before submit
+$ hg revert @99999
 </pre>
 
+<p>
+will revert any files mentioned on CL 99999 to their original state. This can
+be an effective way of reverting one CL revision and applying another.
+</p>
+
+<p>
+Once the CL has been submitted, the next time you run <code>hg sync</code>
+it will be removed from your local pending list. Occasionally the pending list
+can get out of sync leaving stale references to closed or abandoned CLs. 
+You can use <code>hg change -D 99999</code> to remove the reference to CL 99999.
+
+<h3>Submit the change after the review</h3>
+
+<p>
+After the code has been <code>LGTM</code>'ed, it is time to submit
+it to the Mercurial repository. 
+</p>
+
 <p>
 If you are not a committer, you cannot submit the change directly.
-Instead, a committer, usually the reviewer who said <code>LGTM</code>,
+Instead a committer, usually the reviewer who said <code>LGTM</code>,
 will run:
 </p>
 
@@ -486,19 +543,39 @@ $ hg clpatch 99999
 $ hg submit 99999
 </pre>
 
-<p>The <code>clpatch</code> command imports your change 99999 into
-the committer's local Mercurial client, at which point the committer
-can check or test the code more.
-(Anyone can run <code>clpatch</code> to try a change that
-has been uploaded to the code review server.)
 The <code>submit</code> command submits the code.  You will be listed as the
 author, but the change message will also indicate who the committer was.
 Your local client will notice that the change has been submitted
 when you next run <code>hg sync</code>.
 </p>
 
+<p>
+If you are a committer, you can run:
+</p>
+
+<pre>
+$ hg submit 99999
+</pre>
+
+<p>
+This checks the change into the repository.
+The change description will include a link to the code review,
+and the code review will be updated with a link to the change
+in the repository.
+</p>
+
+<p>
+If your local copy of the repository is out of date,
+<code>hg submit</code>
+will refuse the change:
+</p>
+
+<pre>
+$ hg submit 99999
+local repository out of date; must sync before submit
+</pre>
 
-<h3 id="copyright">Copyright</h3>
+<h2 id="copyright">Copyright</h2>
 
 <p>Files in the Go repository don't list author names,
 both to avoid clutter and to avoid having to keep the lists up to date.
@@ -541,3 +618,8 @@ This rigmarole needs to be done only for your first submission.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 </pre>
+
+<p>
+Files in the repository are copyright the year they are added. It is not
+necessary to update the copyright year on files that you change.
+</p>