chore:add PULL_REQUEST_TEMPLATE.md

[Johanna-hub]

(👋 I'm an Outreachy intern at Mozilla and assisting @marcoscaceres with specifications)

During TPAC it was proposed to adopt something closer to the WHATWG Working Mode. That is, before merging a pull request, the following things should be in place:

• Web Platform tests for the feature/fix being proposed.
• Explicit implementer commitment to add/fix whatever is being proposed.

This template provides some simple checkboxes to make sure the above conditions are met. We strongly encourage using this template, since it helps implementers find tests and sends a strong signal about implementation commitment. You are free to modify it to better suit your needs (see an example of a modified template).

If you are unsure how to create Web Platform Tests, there are some great docs available. We are also here to help you get started if you need guidance.

[LJWatson]

This spec doesn't use WPT because it documents author conformance, not browser implementations.

[Johanna-hub]

ah, sorry I'll close.