Add Code of Conduct

Summary:
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.

Adding this file will improve the [Infer community profile](https://github.com/facebook/infer/community) checklist and increase the visibility of our COC.

Reviewed By: jvillard

Differential Revision: D6724173

fbshipit-source-id: 88e6f37

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/pages/876921332402685/open-source-code-of-conduct) so that you can understand what actions will and will not be tolerated.

CONTRIBUTING.md

@@ -61,8 +61,11 @@ repository. To change that part of the code:
 We require contributors to sign our Contributor License Agreement. In
 order for us to review and merge your code, please sign up at
 https://code.facebook.com/cla. If you have any questions, please drop
-us a line at cla@fb.com. Thanks!
+us a line at cla@fb.com.
 
+You are also expected to follow the [Code of Conduct](CODE_OF_CONDUCT.md), so please read that if you are a new contributor.
+
+Thanks!
 
 ## Coding Style