Add `CODE_OF_CONDUCT`
Add `CODE_OF_CONDUCT`

**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.

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 improve the [prop-types Community Profile](https://github.com/facebook/prop-types/community)
checklist and increase the visibility of our COC.

**test plan:**
Viewing it on my branch -
(Flarnie will insert screenshots)

**issue:**
internal task t23481323

reactjs/prop-types,facebook/prop-types