Add Code of Conduct (#231)

* Add link to COC in `CONTRIBUTING.md`

We are about to add a COC and it makes sense to surface it here.

* Add `CODE_OF_CONDUCT.md`

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

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

**issue:**
internal task t23481323

Author: flarnie

CODE_OF_CONDUCT.md

@@ -0,0 +1,3 @@
+# Code of Conduct
+
+Facebook has adopted a Code of Conduct that we expect project participants to adhere to. Please [read the full text](https://code.facebook.com/codeofconduct) so that you can understand what actions will and will not be tolerated.

CONTRIBUTING.md

@@ -2,6 +2,9 @@
 We want to make contributing to this project as easy and transparent as
 possible.
 
+## Code of Conduct
+The code of conduct is described in [`CODE_OF_CONDUCT.md`](CODE_OF_CONDUCT.md)
+
 ## Our Development Process
 The majority of development on jscodeshift will occur through GitHub. Accordingly,
 the process for contributing will follow standard GitHub protocol.