add CONTRIBUTING guidelines to repo

Simply create a document called CONTRIBUTING.md in the root of the repository, use it to describe the preferred issue reporting workflow, and

Make sure to emphasize the importance of submitting issues for anything that the user does not understand.

[Feel free to draw inspiration from here](https://github.com/microsoft/vscode/blob/cf4c4a0ece70d221d907a2bc0042cc164f73bb0c/CONTRIBUTING.md)

You can also **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.

Create CONTRIBUTING.md at the repository root. Start by reviewing the linked VS Code contributing guide and the React contribution writeup, then adapt the issue-reporting workflow to this repository. Done means the document explains how to report issues, including reporting anything users do not understand.

Written by the indexing model from the issue text.