Add a section about "things to keep in mind when reviewing a PR".

That is a good point, although we devs tend to have a hard time knowing or remembering what normies know and don't know. It's definitely something to keep in mind when reviewing other devs' pull requests.

This triggered a thought: Should we have a document in our dev guide that talks about reviewing PRs and has a list of thangs like this to keep in mind? I think such a document could be useful, but could also turn into a list that is too long and random to be of any use. Does 18F or TTS have a document we can borrow/steal?

_Originally posted by @jsf9k in https://github.com/cisagov/skeleton-docker/pull/51#r609700136_