Add CONTRIBUTING.md docs

[enijar]

I'm attempting to add a new feature to this library: visual scrollbars. Can you add a CONTRIBUTING.md file that outlines how developers can get up and running with contributing to this library? Please also let me know if this is something you would accept as a PR to this library.

So far I've gathered the getting started steps to be:

npm install
npm run build
npm --prefix tests/manual install
npm --prefix tests/manual run dev

However, it's a rather inconvenient DX to have to keep running npm run build from the project root just to see changes reflected in tests/manual.