Add a `CONTRIBUTING.md` file

[MarkBennett]

We should probably add a CONTRIBUTING.md file...

Originally posted by @lucacasonato in https://github.com/denoland/fresh/issues/335#issuecomment-1171772496

This came up for me as I was trying to write my first contribution to Fresh but couldn't get the tests to run until I realized that deno-puppeteer needed to be installed first.

Happy to make a PR if there's interest but wanted to post an issue first.