Add contributing instructions

[danijar]

Recommended by user unbalancedparen on Twitter. We should add a CONTRIBUTING.md to the repository and roughly states:

• Contributions are welcome
• Check out the Github issues and join the discussions
• Comment on an issue with "help wanted" tag to discuss potential solutions
• Implement the feature and send a PR
• The is a discussion tag that can be used for discussion features or meta discussions (this acts as our forum/slack channel)

Anything else I should mention?

[epogrebnyak]

Code style preferences and maybe autopep config?