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a PR should contain more information as described in the changes below to make for easier/better reviews as well as for historical reference.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.
what do you think about making this JIRA suggestion stronger? in the land of best practices, there would always be a JIRA ticket corresponding to a change, so maybe all of these examples could have a JIRA key in them?