GitHub Contributing Wiki Page

[Gabe Alford]

Hello,

I have added a wiki page for contributing on GitHub for our
project here
https://github.com/OpenSCAP/scap-security-guide/wiki/Contributing. Please
let me know what you think and what needs to be added/deleted. If the
fedorahosted wiki is what needs to be updated and used for a wiki instead
of GitHub, I can do that as well.

Thanks,

Gabe