Initial stab at CONTRIBUTING.md for puppet module

Basing this off of alphagov/business-support-finder#46 by @mnowster

I explicitly haven't linked to our puppet style guide as that's got a
lot of stuff that's not relevant for modules (nagios, upstart etc).
Should just need one command to run all tests
Merge pull request #12 from gds-operations/contributing

Initial stab at CONTRIBUTING.md for puppet module