add CODING_GUIDELINES.md

CODING_GUIDELINES.md

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+# Coding Guidelines
+
+This is a live document that collects those coding guidelines we intend new code
+to follow.
+
+
+## ESLint
+
+- Run `yarn lint` to validate your code.
+
+- Don't commit code with ESLint errors (it'll make the build in CircleCI fail).
+
+- Code with ESLint warnings won't make the build in CircleCI fail, but reviewers
+  will likely request they're fixed before merging a contribution.
+
+
+## Naming Conventions
+
+### Path and Filenames
+
+- Use lower case names and only alphanumeric characters (unless required
+  otherwise).
+
+- Avoid stuttering:
+  - bad: `my-connector/my-connector-driver.js`
+  - good: `my-connector/driver.js`
+
+- Use the extension `.jsx` for files that contain JSX and `.js` for those that
+  only contain JavaScript.
+
+### Variable and Function Names
+
+- Use JavaScript convention, i.e. camelCase.