Update ContributingExample and move into docs

The wiki page https://github.com/rapidsms/rapidsms/wiki/ContributingExample gives great step-by-step instructions for submitting a bug fix to RapidSMS. Unfortunately it's out of date with regard to the repositories. It should be possible to simplify it a bit.

Also, let's move it into the Contributing section of the documentation where it's more likely that people who might contribute will come across it.