Add a PR template

[aliddell]

Suggestions welcome. Should go in all the repos once it's approved.

[andy-sweet]

As discussed in standup, this is good start, definitely better than the nothing we have now, and we can iteratively improve this as needed.

I made a few comments, some of which might be worth addressing, but none of which are blocking.

[aliddell]

On closer consideration, you @andy-sweet and @nclack were right that putting changes here in changelog format are absolutely duplication of effort. If the submitter has checked all the boxes, the reviewer should be able to tell from looking at the changelog, if nothing else, what's changed, and in that exact format.