Update contributing guidelines especially around PRs

[joe94]

I'm not quite sure how stable our current PR guidelines will stay around what needs to be done to document changes, make releases, etc. But today, we just ask for the changelog.md file to be updated with the following guidelines:

• (Contributors should consult maintainers before doing this) Add a version section in the changelog file and leave the unreleased section empty if a new version release is to be made. Example: 96545c8

• If no release needs to be made immediately, simply add a change entry with the link to the PR following changelog guidelines when possible https://keepachangelog.com/en/1.0.0/#how