Create CONTRIBUTE.md guide
It would be neat to have some documentation about how to contribute and develop for this project.

Points to mention: 

- [ ] Make sure to use `prettier` as a code formatter
- [ ] Open PRs to the `next` branch instead of `master` (that will simplify the publishing workflow). We will also need to change it in the repo settings, but it's good to have it documented (kind of like in https://github.com/homebridge/HAP-NodeJS/wiki/Release-Cycle#git-workflow)