[docs] Document the commit message format

Clutter uses the standard git format for commit messages:

        short description
        <newline>
        long description...

And everyone asking for their patches or trees to be merged upstream
should conform to the same commit messages format, just like they
should conform to the coding style.

HACKING

@@ -58,6 +58,21 @@ General notes and rules on clutter core hacking;
    tests/conform and, eventually, a user-interactive tes under
    tests/interactive.
 
+ - When committing, use the standard git commit message format:
+
+     short description          - MUST be less than 74 characters
+     <newline>                  - MANDATORY empty line
+     long description           - Each line must be less than 80 characters
+
+   Do NOT put the commit message on the short description line.
+   One line commit messages should be avoided, unless they can be
+   *fully* explained in less than 70 characters (e.g. "Fix typo in
+   clutter_actor_create_pango_context() docs"). Think of the commit
+   message as an email sent to the maintainers explaining "what" you
+   did and, more importantly, "why" you did it. The "how" is not
+   important, since "git show" will show the patch inlined with the
+   commit message.
+
 RELEASES
 ========