Add a better Contributing guideline

Created by: gaearon

We should explain:

• Constraints (e.g. no flags, no configuration)
• Strategies (find good defaults, good heuristics can make)
• Actively find ways to improve DX (e.g. remove noise, implement things you didn’t see in other tools but wish were there)
• Try to make the least controversial choices (would that work for 95% of users? would beginners understand this?)
• Coordinate on an issue in case somebody else is already working on the PR you want
• If you submit a PR please provide a test plan (how did you verify it works?)
• Release guidelines (how to run the script, update changelog etc)