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a PR should contain more information as described in the changes below to make for easier/better reviews as well as for historical reference.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.
Done in 40a47486af115c07aff3c746d77e5f687206fa67