Add suggestion and examples of imperative text

Add grammar suggestions for consistent git messages.  Following the same
mood makes generation of release notes and working in Github in general
easier.

Based on comments found at:
http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html

.gitmessage

@@ -5,7 +5,9 @@ a commit message. To use this template, change the first line (think of
 it as a "headline" for the commit) and then edit this message with a
 longer description of the change.  To be nice, keep lines to 72
 characters or less (The first two lines of this template are exactly
-that length).
+that length).  Also use imperative writing, e.g. "Fix broken code" or
+"Implement my new feature", not "Fixes the broken code" or
+"Implemented my new feature".
 
 Sections below can then be filled out and edited as appropriate.
 Unused sections can optionally be removed.