Add a pull request template

[aeisenberg]

This PR adds a pull request template. It is sparse right now. The main goal is to remind query authors to check for autofix changes when they add new queries or update existing ones.

[aeisenberg]

I'm sure we can fill this out with a lot more. For now, though, I just want a non-intrusive way to remind about autofix. Suggestions on other things to add would be appreciated.

[aeisenberg]

Adding codeql Engineering Managers as reviewers since we discussed this PR this morning.

[oscarsj]

mentioning @esbena in case this was lost in the notifications maelstrom

[aeisenberg]

I'll keep this open until Tuesday so I can give others a chance to review.