(Maint) Clarify which branches changes should be based on

Priror to this commit, CONTRIBUTING.md did not reflect our current
process in regards to where changes should be targeted. Now it is
clear on these issues so that it will be easier for community
members to target their submissions to the right location, and
prevent us from frequently having to kick back pull requests that
need to be rebased and retargeted.

<hailee@puppetlabs.com>