Add "How to Contribute" to README

[r00k]

My fork: http://github.com/r00k/github-gem

I've added a "How to Contribute" section to the README. I got the 'issues instead of pull requests' from defunkt's auto-responder when I tried to send a pull request.

Also, I removed the instructions to add gemcutter as a source, as that's no longer necessary.