Add setup_commit_msg_hook.sh

This script can be used by developers to add a commit-msg hook to their repo.
It comes from discussions in issue #207.

This commit-msg hook will automatically add the following 2 trailers at the end of each commit message:

License: MIT
Signed-off-by: User Name <user.email@example.com>

There are at least the following points to consider before merging this PR:

- The script setup_commit_msg_hook.sh should probably be moved inside a subdirectory instead of being at the root of the repo.
- There should be some documentation explaining why developers should sign their work and how the script can help.  
- Maybe it's better to add these trailers before the user fills up the commit message instead of after.