update contributing to reflect modules workflow

CONTRIBUTING has been dropped in from puppetlabs/puppet, with
good intention I assume, however it does in no way reflect how
we work on modules.

* change all references from Redmine to refer to GitHub
* Fix CLA links
* remove the entire section for committers, we assume they know what
they are doing...

Further, we remove all contractions. Why? Because it reads easier.

Finally we will need a section on how to write tests (and run them
locally) because I'm getting sick of repeating myself.