Add some information on must haves in a PR description.

Right now, a lot of PRs contain only information about the changes being made like "Add foo module" but not enough context around the change. Ideally, a PR should contain more information as described in the changes below to make for easier/better reviews as well as for historical reference.

Tested the formatting by looking at the rendered markdown on github.
regarding helpful to the reviewer: I'd really like to see us have a convention of putting PR links in the JIRA ticket + JIRA links in PRs to minimize a reviewer's work... 

There are actually tools that will do all this for you in JIRA though, we could just enable them...