Summary:
Hi! I noticed this repo could use some documentation to help new contributors.

**why make this change?:**
Regarding the `CONTRIBUTING.md` -> Including instructions for new contributors to open source is a courtesy that will foster a welcoming and safe open source community at Facebook.

Regarding the Code of Conduct ->

Facebook Open Source provides a Code of Conduct statement for all
projects to follow, to promote a welcoming and safe open source community.

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 folly community profile](https://github.com/facebook/folly/community)
checklist and increase the visibility of our COC.

Closes https://github.com/facebook/folly/pull/719

Reviewed By: flarnie

Differential Revision: D6881011

Pulled By: simpkins

fbshipit-source-id: a6f0988ac2b49f81bb13d3219095105b7b27e66f