[Github Community] Add a PR template
Add a PR template to reduce back and forth conversations between contributors and reviewers, making merging external pull requests life easier.
The Contributing.md will create a pop-up like below once a contributor create a new PR:
Test Plan: The template can only be seen after landing on master. Fortunately, it doesn't affect anything else.
Reviewers: alangenfeld, max, catherinewu
Reviewed By: catherinewu
Subscribers: alangenfeld, catherinewu
Differential Revision: https://dagster.phacility.com/D5651