[Github Community] Add a PR template

Summary:
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:

Reference: open-source-templates

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