docs: Add contribution guide (#916)

Author: mbfakourii

CONTRIBUTING.md

@@ -0,0 +1,23 @@
+# Contributing to Parse SDK for Flutter
+
+We want to make contributing to this project as easy and transparent as possible.
+
+## Pull Requests
+
+We actively welcome your pull requests. When we get one, we'll run some Parse-specific integration tests on it first. From here, we'll need to get a core member to sign off on the changes and then merge the pull request. For API changes we may need to fix internal uses, which could cause some delay. We'll do our best to provide updates and feedback throughout the process.
+
+1. Fork the repo and create your branch from `master`.
+2. Add unit tests for any new code you add.
+3. If you've changed APIs, update the documentation.
+4. Ensure the test suite passes.
+
+## Code of Conduct
+
+This project adheres to the [Contributor Covenant Code of Conduct](https://github.com/parse-community/parse-server/blob/master/CODE_OF_CONDUCT.md). By participating, you are expected to honor this code.
+
+## License
+
+By contributing to Parse SDK Flutter, you agree that your contributions will be licensed under its license.
+
+[stack-overflow]: http://stackoverflow.com/tags/parse.com
+[bug-reports]: https://github.com/parse-community/parse-server