Add guidelines for commit messages

From git-commit(1)

docs/development.pod

@@ -52,10 +52,19 @@ When you're ready to check it in...
   git add lib/Qpsmtpd.pm     # to let git know you changed the file
   git add --patch plugin/tls # interactive choose which changes to add
   git diff --cached          # review changes added
-  git commit
+  git commit                 # describe the commit
   git log -p                 # review your commit a last time
   git push origin            # to send to github
 
+=head3 Commit Descriptions
+
+Though not required, it's a good idea to begin the commit message with
+a single short (less than 50 character) line summarizing the change,
+followed by a blank line and then a more thorough description. Tools
+that turn commits into email, for example, use the first line on the
+Subject: line and the rest of the commit in the body.
+(From: L<git-commit(1)>)
+
 =head3 Submit patches by mail
 
 The best way to submit patches to the project is to send them to the