Add CONTRIBUTING.md

Author: davidliu

CONTRIBUTING.md

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+# Contributing
+We appreciate any pull requests for changes you may have in mind! Here are some tips and instructions you should follow before we can accept your PR.
+
+* Add a changeset file which explains the changes contained in the PR.
+
+  In the root folder, execute the following commands:
+  ```
+  pnpm install
+  pnpm changeset
+  ```
+
+  Follow the instructions on screen to create the changeset file.
+
+* Format your code using `./gradlew spotlessApply`.
+
+* On your first pull request, the CLA Assistant bot will give you a link to sign this project's Contributor License Agreement, required to add your code to the repository. This license is non-optional and we cannot accept any PRs from contributors who have not signed the CLA.
+