doc: add notes on good commit messages

Explain Brad's algorithm for generating commit headlines.

Fixes #15700

Change-Id: Ic602f17629b3dd7675e2bb1ed119062c03353ee9
Reviewed-on: https://go-review.googlesource.com/23355Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

doc/contribute.html

@@ -353,10 +353,13 @@ with a thorough description of your change.
 The first line of the change description is conventionally a one-line
 summary of the change, prefixed by the primary affected package,
 and is used as the subject for code review mail.
-The rest of the
-description elaborates and should provide context for the
+It should complete the sentence "This change modifies Go to _____."
+The rest of the description elaborates and should provide context for the
 change and explain what it does.
+Write in complete sentences with correct punctuation, just like
+for your comments in Go.
 If there is a helpful reference, mention it here.
+If you've fixed an issue, reference it by number with a # before it.
 </p>
 
 <p>
@@ -364,7 +367,7 @@ After editing, the template might now read:
 </p>
 
 <pre>
-math: improved Sin, Cos and Tan precision for very large arguments
+math: improve Sin, Cos and Tan precision for very large arguments
 
 The existing implementation has poor numerical properties for
 large arguments, so use the McGillicutty algorithm to improve