contributing guide / pkg.pr.new support

[benmccann]

We've got some code changes that we wanted to PR, but I wanted to better understand how to test things how locally. It's a little tricky since you need experience with both JS and C++ and my skill level is very different between those two. If there was a small contributors guide with some pointers it would really help

Another idea I wanted to share was adding support for pkg.pr.new which lets you publish packages for testing. You can use a GitHub Actions workflow to publish a package with every commit on a PR and then anyone can grab that version for testing out, which would help with manual testing