Add coding standards

Summary:
Add some expectations for external contributions.

Original Author: [email protected]
Original Git: 869312f

Original Reviewed By: avp

Original Revision: D46136240

Reviewed By: neildhar

Differential Revision: D47690407

fbshipit-source-id: 3e63a25a00e7952b9ec438b055dd846f2f89da6b

Author: avp

CONTRIBUTING.md

@@ -1,6 +1,9 @@
 # Contributing to Hermes
 We want to make contributing to this project simple and convenient.
 
+This document provides a high level overview of the contribution process,
+please also review our [Coding Standards](doc/CodingStandards.md).
+
 ## 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.fb.com/codeofconduct/) so

doc/CodingStandards.md

@@ -9,6 +9,18 @@ This document provides guidance for the kind of code that should go in to
 the Hermes project. The rules in this document will allow us to scale the
 project and ensure that the code base remains readable and maintainable.
 
+### Code Quality
+
+Code quality standards in Hermes are very strict. While we welcome and
+support community contributions, expect scrutiny on changes that do not
+meet the following standards:
+
+- Contributions must be well understood and explained in PRs. We cannot accept
+and maintain contributions that we don't fully understand.
+- Contributions should not use shortcuts or hacks in the name of expediency.
+Hermes is a critical piece of infrastructure for many apps and systems, and
+such code creates ongoing work for maintainers.
+
 ### Language Features
 
 Hermes uses C++14.