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a PR should contain more information as described in the changes below to make for easier/better reviews as well as for historical reference.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.
This is awesome. Maybe you could make a dedicated section for Pull Requests above (like there is for Branching and Committing)? That way the Workflow section remains focused on git actions and the sections above describe style.