Added CONTRIBUTING.md doc

Author: mpeyper

CONTRIBUTING.md

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+# Contributing
+
+Thanks for wanting to make contribution and wanting to improve this library for everyone!
+
+## Project setup
+
+1.  Fork and clone the repo
+2.  Run `npm install` to install dependencies
+3.  Create a branch for your PR with `git checkout -b pr/your-branch-name`
+
+## Add yourself as a contributor
+
+This project follows the [all contributors](https://github.com/kentcdodds/all-contributors) specification. To
+add yourself to the table of contributors on the `README.md`, please use the
+automated script as part of your PR:
+
+```console
+npm run contributors:add
+```
+
+Follow the prompt and commit `.all-contributorsrc` and `README.md` in the PR. If
+you've already added yourself to the list and are making a new type of
+contribution, you can run it again and select the added contribution type.
+
+## Committing and Pushing changes
+
+Please make sure to run the tests before you commit your changes. You can do so by running
+`npm test`.
+
+### Add typings
+
+If your PR introduced some changes in the API, you are more than welcome to
+modify the Typescript type definition to reflect those changes. Just modify the
+`index.d.ts` file accordingly. If you have never seen Typescript
+definitions before, you can read more about it in its
+[documentation pages](https://www.typescriptlang.org/docs/handbook/declaration-files/introduction.html)
+
+## Help needed
+
+Please checkout the [the open issues](https://github.com/mpeyper/react-hooks-testing-library/issues)
+
+Also, please watch the repo and respond to questions/bug reports/feature
+requests! Thanks!