Snaps contributing guide

The extension repo has a rich guide for contributors: https://github.com/MetaMask/metamask-extension#contributing

A similar guide should be written for Snaps, with the following information:

- How to get set up and start testing
- What to know when submitting a PR
- Architecture of Snaps
- Style guide for new APIs
- Refer to SIPs process
- Documentation requirements and refer to docs repo

The end goal is anyone should be able to build new Snaps APIs and submit them without having to get training from the Snaps team.

The current contributing guide covers the first part, but doesn't cover the rest.