Add notes about git commit messages

This improves readability of commits a lot and is easy to
follow. I think most people follow those rules already. They are based
on http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html

Docker-DCO-1.1-Signed-off-by: Johannes 'fish' Ziemke <github@freigeist.org> (github: discordianfish)
Adding Rohit Jnagal and Victor Marmol to pkg/libcontainer maintainers.

Docker-DCO-1.1-Signed-off-by: Victor Marmol <vmarmol@google.com> (github: vmarmol)
Merge pull request #5536 from vmarmol/add-maintainers-libcontainer
Merge pull request #5501 from discordianfish/add-commit-message-bp