doc: Mention contributor agreement immediately after Gerrit

I walked through the steps for a contribution but ended up
with an error when doing "git mail" because I didn't have a
signed agreement.

Added a section to check for or create one through Gerrit right
after the user has created the account and logged in.

Moved some info from copyright section to the new section.

Change-Id: I79bbd3e18fc3a742fa59a242085da14be9e19ba0
Reviewed-on: https://go-review.googlesource.com/13062Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>

doc/contribute.html

@@ -24,7 +24,7 @@ you're working on if you want to change or add to the Go repositories.
 </p>
 
 <p>
-Before undertaking to write something new for the Go project, 
+Before undertaking to write something new for the Go project,
 please <a href="https://golang.org/issue/new">file an issue</a>
 (or claim an <a href="https://golang.org/issues">existing issue</a>).
 Significant changes must go through the
@@ -124,6 +124,41 @@ to Google Accounts. Sign in using the same Google Account you used above.
 That is all that is required.
 </p>
 
+<h3>Contributor License Agreement</h3>
+
+<p>Gerrit serves as the gatekeeper and uses your e-mail address as the key.
+To send your first change to the Go project from a given address,
+you must have completed one of the contributor license agreements:
+<ul>
+<li>
+If you are the copyright holder, you will need to agree to the
+<a href="https://developers.google.com/open-source/cla/individual">individual
+contributor license agreement</a>, which can be completed online.
+</li>
+<li>
+If your organization is the copyright holder, the organization
+will need to agree to the
+<a href="https://developers.google.com/open-source/cla/corporate">corporate
+contributor license agreement</a>.
+(If the copyright holder for your code has already completed the
+agreement in connection with another Google open source project,
+it does not need to be completed again.)
+</li>
+</ul>
+
+<p>
+You can use the links above to create and sign the contributor license agreement
+or you can show your current agreements and create new ones through the Gerrit
+interface.  <a href="https://go-review.googlesource.com/login/">Log into Gerrit</a>,
+click your name in the upper-right, choose "Settings", then select "Agreements"
+from the topics on the left. If you do not have a signed agreement listed here,
+you can create one by clicking "New Contributor Agreement" and following the steps.
+</p>
+
+<p>
+This rigmarole only needs to be done for your first submission for each email address.
+</p>
+
 <h3>Install the git-codereview command</h3>
 
 <p>
@@ -628,30 +663,6 @@ the <a href="/AUTHORS"><code>AUTHORS</code></a> file defines
 who &ldquo;The Go Authors&rdquo;&mdash;the copyright holders&mdash;are.
 These files will be periodically updated based on the commit logs.
 
-<p>Gerrit serves as the gatekeeper and uses your e-mail address as the key.
-To send your first change to the Go project from a given address,
-you must have completed one of the contributor license agreements:
-<ul>
-<li>
-If you are the copyright holder, you will need to agree to the
-<a href="https://developers.google.com/open-source/cla/individual">individual
-contributor license agreement</a>, which can be completed online.
-</li>
-<li>
-If your organization is the copyright holder, the organization
-will need to agree to the
-<a href="https://developers.google.com/open-source/cla/corporate">corporate
-contributor license agreement</a>.
-(If the copyright holder for your code has already completed the
-agreement in connection with another Google open source project,
-it does not need to be completed again.)
-</li>
-</ul>
-
-<p>
-This rigmarole needs to be done only for your first submission.
-</p>
-
 <p>Code that you contribute should use the standard copyright header:</p>
 
 <pre>