Add document describing the RN ecosystem. (#24493)

Summary:
This change adds a document outlining the main stakeholders in React Native and their responsibilities: Partners, core contributors and community contributors

It serves as an overview for companies and individuals who are interested in contributing, to get a better understand of how the project is set up, who supports it, and what they do.

This is the first version and I am hoping to integrate it into more places in the future as well as I'd like to keep it up-to-date as things change. Hector is working on a detailed document outlining what it means to be a core contributor and we'll make a list of existing core contributors soon as well.
Pull Request resolved: https://github.com/facebook/react-native/pull/24493

Differential Revision: D14973910

Pulled By: cpojer

fbshipit-source-id: 3a722582be6bb3b953f10073682c5841cdae2c27