_content/doc: recommend wrapping commit message text

Change-Id: Icf39a78b3d04c81fb6a36a9b23a3b9586a6a6964
Reviewed-on: https://go-review.googlesource.com/c/website/+/727860
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Florian Lehner <lehner.florian86@gmail.com>
Reviewed-by: t hepudds <thepudds1460@gmail.com>

Author: seankhliao

_content/doc/contribute.html

@@ -815,6 +815,8 @@ <h3 id="main_content">Main content</h3>
 Write in complete sentences with correct punctuation, just like
 for your comments in Go.
 Don't use HTML, Markdown, or any other markup language.
+The text should be wrapped at around 72 columns.
+See <a href="/wiki/CommitMessage">Commit messages</a> for additional details.
 </p>
 
 <p>