add CONTRIBUTING section to README

Supposing the contributing guidelines have already been created, we need to improve the readme to link to that information

**If it has not yet been created check issue #3 and tackle that first**

USE THE FOLLOWING WRITEUP AND ADAPT ACCORDINGLY THEN ADD TO THE END OF THE README

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.

[Code of Conduct](https://code.fb.com/codeofconduct)
Facebook has adopted a Code of Conduct that we expect project participants to adhere to. Please read [the full text](https://code.fb.com/codeofconduct) so that you can understand what actions will and will not be tolerated.

[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.

[Good First Issues](https://github.com/facebook/react/labels/good%20first%20issue)
To help you get your feet wet and get you familiar with our contribution process, we have a list of [good first issues](https://github.com/facebook/react/labels/good%20first%20issue) that contain bugs that have a relatively limited scope. This is a great place to get started.

License
React is [MIT licensed](https://github.com/facebook/react/blob/main/LICENSE).

First check issue #3 to confirm that the contributing guidelines exist. Then open the repository README, adapt the provided Contributing writeup, and add it to the end. Done means the README contains the requested Contributing section and its links, including the Code of Conduct, Contributing Guide, Good First Issues, and License information.

Written by the indexing model from the issue text.