**what is the change?:**
Adding a document linking to the Facebook Open Source Code of Conduct,
for visibility and to meet Github community standards.
**why make this change?:**
Facebook Open Source provides a Code of Conduct statement for all
projects to follow.
React already links to this Code of Conduct in the README, which is
great!
Exposing the COC via a separate markdown file is a standard being
promoted by Github via the Community Profile in order to meet their Open
Source Guide's recommended community standards.
As you can see, adding this file will complete [React's Community Profile](https://github.com/facebook/react/community)
checklist and increase the visibility of our COC.
**test plan:**
Viewing it on my branch -
(Flarnie will insert screenshot)
**issue:**
internal task t23481323
