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a PR should contain more information as described in the changes below to make for easier/better reviews as well as for historical reference.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.
I agree that it would be the case in the ideal world, but in practice we would always have changes without a JIRA ticket. I would prefer something that is more reflect of reality. That said, I don't feel too strongly about adding JIRA keys in all of the examples.