add CONTRIBUTING section to README

[balotofi]

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
Facebook has adopted a Code of Conduct that we expect project participants to adhere to. Please read the full text so that you can understand what actions will and will not be tolerated.

Contributing Guide
Read our contributing guide 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
To help you get your feet wet and get you familiar with our contribution process, we have a list of good first issues that contain bugs that have a relatively limited scope. This is a great place to get started.

License
React is MIT licensed.