Add coding standards

Summary: Add some expectations for external contributions.

Reviewed By: avp

Differential Revision: D46136240

fbshipit-source-id: 22f96a67ac74ab00af75e557cac700cfc95ddf0d

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.