Add commit messages guideline

Style comes from git itselfs
(http://git.kernel.org/cgit/git/git.git/tree/Documentation/SubmittingPatches?id=HEAD)

Lines 110-166

```
Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
to do frotz", as if you are giving orders to the codebase to change
its behaviour.  Try to make sure your explanation can be understood
without external resources. Instead of giving a URL to a mailing list
archive, summarize the relevant points of the discussion.
```