Start working on a better CONTRIBUTING.md

[BinderDavid]

Fixes #364

Rendered CONTRIBUTING.md: https://github.com/polarity-lang/polarity/blob/c9be5ea0c5e9bc45a6a78f8d7a01fd6c9f915f61/CONTRIBUTING.md Rendered README.md: https://github.com/polarity-lang/polarity/blob/c9be5ea0c5e9bc45a6a78f8d7a01fd6c9f915f61/README.md

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

see 5 files with indirect coverage changes

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🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests
• JS Bundle Analysis - Avoid shipping oversized bundles

[BinderDavid]

I like this. Maybe you can also add a small sentence containing something like "If you are a nix user, please refer to contrib/nix/README.md"?

Done in cb7506b48b9b924cb8fb965f7c965e34dd12e0f8 :)