add contributing guidelines to repo

**TAKE INSPIRATION** FROM THE FOLLOWING WRITEUP AND ADAPT ACCORDINGLY

Contributing
The main purpose of this repository is to continue evolving React core, making it faster and easier to use. Development of React happens in the open on GitHub, and we are grateful to the community for contributing bugfixes and improvements. Read below to learn how you can take part in improving React.

[Contributing Guide](https://legacy.reactjs.org/docs/how-to-contribute.html)
Read our [contributing guide](https://legacy.reactjs.org/docs/how-to-contribute.html) to learn about our development process, how to propose bugfixes and improvements, and how to build and test your changes to React.

Start by reviewing the repository root and any existing documentation, then adapt the linked React contributing-guide inspiration to this satellite documentation project. Done means a repository-specific contributing guide explains how contributors can participate, propose changes, and build or test changes where applicable.

Written by the indexing model from the issue text.