(doc) Sync latest CONTRIBUTING.md
(doc) Sync latest CONTRIBUTING.md

This is the latest CONTRIBUTING.md from
https://github.com/puppetlabs/puppet/blob/master/CONTRIBUTING.md

The old version has out of date links.

ianand0204/puppetlabs-firewall,puppetlabs/puppetlabs-firewall,jaakkosipari/puppetlabs-firewall