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.

Review the linked React contributing guide and inspect the repository structure to determine where project guidance belongs. Adapt the relevant contribution, build, and testing information for this repository; done means a clear contributing guide is present and accurately describes how contributors can participate.

Written by the indexing model from the issue text.