Convert CONTRIBUTING to markdown and some updates

This converts the original CONTRIBUTING document to markdown for better
rendering on github.

Additionally this removes some wording which was e-mail/patch specific
and instead refers to changes as contributions or commits.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Merge pull request #11 from stgraber/master

Convert CONTRIBUTING to markdown and some updates