Update CONTRIBUTING.md to provide more guidance

[hanneshapke]

List out

• How to set up a dev environment
• How to test the code
• Set up Code ownership

@codesue made the great suggestion to model the CONTRIBUTING.md according to tfxa.

[codesue]

Oh, I didn't mean updating the whole contributing guide. I meant including a code style section with an overview of our conventions and instructions on how to lint code locally as part of or after #242, and that we could use tfx and tfxa as starting points.

We can take a more incremental and iterative approach, e.g. adding information about testing when that process is set up, using a separate PR for code ownership since that would require more discussion and have blocking reviewers, etc.

That said, I really like 🤗 Transformers' CONTRIBUTING.md as a general model.

[codesue]

Completed in #259