Fix missing title mark in spec

The "remote operations" section wasn't marked as such in markdown,
leading to a rather confusing rendering.

Reported-by: Dwight Engen <dwight.engen@oracle.com>
Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
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>