docs(contributing): add CONTRIBUTING.md

Author: notrab

CONTRIBUTING.md

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+# Contributing to `react-use-cart`
+
+This library was built to be a thin layer between your product inventory + checkout.
+
+Before you get started writing any code, make sure:
+
+- You search open PRs and issues, someone else might be working on it
+- Adhere to [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/) - this helps out our automated release bot
+- Update the `README` with any added functionality usage instructions
+- Add test coverage
+
+## Working in development
+
+This library uses [tsdx](https://tsdx.io). Execute `npm start` to start the project in development/watch mode.
+
+The example directory uses the library with parcel, and you'll need to `cd example` and `npm install` to get going.
+
+## PR early
+
+The single most important thing you can do is PR early in `DRAFT` status. This allows maintainers, and other users to see what you're working on, and provide any suggestions early.
+