Summary:
In the past Facebook didn't promote including a Code of Conduct when creating new projects, and many projects skipped this important document. Let's fix it. :)

**why make this change?:**
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 rocksdb community profile](https://github.com/facebook/rocksdb/community)
checklist and increase the visibility of our COC.

**test plan:**
Viewing it on my branch -
<img width="1008" alt="screen shot 2017-12-03 at 5 05 45 pm" src="https://user-images.githubusercontent.com/1114467/33532198-66012a56-d84c-11e7-8fab-29ed410bd600.png">
<img width="1015" alt="screen shot 2017-12-03 at 5 05 59 pm" src="https://user-images.githubusercontent.com/1114467/33532199-661813d8-d84c-11e7-941e-94754dd481e5.png">

**issue:**
internal task t23481323
Closes https://github.com/facebook/rocksdb/pull/3219

Reviewed By: yiwu-arbug

Differential Revision: D6494234

Pulled By: flarnie

fbshipit-source-id: 55b59db335cc5546f3a1c968322b9281a3dc3aaf