Write documentation for PULL_REQUESTS.md

[mnyon-grandkru]

We need to create documentation for PULL_REQUESTS.md. In order to do that, we need to decide on how we're handling pull requests. The qg-frontend-v2 repo is using a version of GitHub flow which includes a pull request per issue. Other items to consider:

• Enforcing stricter status checks for pull requests
• Whether we make people have Verified commits with SSH or GPG keys for pull requests.

[Mekebrown]

A draft of a pull request template can be found in the general templates document It is currently under review. Hopefully this draft can fulfill this issue.