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a PR should contain more information as described in the changes below to make for easier/better reviews as well as for historical reference.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.
just noticed the plural here should probably be singular, i.e. "All pull request descriptions should contain the following things"

Although I suggest "The description of a pull request must contain the following things:"