Add coding-guidelines.md

docs/coding-guidelines.md

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+When possible, the following are enforced through the code formatter
+(Prettier.js) and tslint rules.
+
+----
+
+## Indentation
+
+We use spaces, not tabs.
+
+## Names
+
+* Use PascalCase for `type` names
+* Use UPPERCASE_WITH_SPACES for `enum` values and constants
+* Use camelCase for `function` and `method` names
+* Use camelCase for `property` names and `local variables`
+* Use whole words in names when possible
+* Use camelCase for file names (name files after the main Type it exports)
+
+## Conventions
+
+* Create a folder for each major subarea
+* In the folder, create an index.ts which exports the public facing API for that
+  subarea.
+* Tests can refer directly to the .ts files; other consumers should refer to the
+  index.ts file.
+
+## Comments
+
+* Use sparingly since comments always become outdated quickly.
+* If you must, use JSDoc style comments.
+
+## Strings
+
+* Use 'single quotes'
+* All strings visible to the user need to be externalized in a `messages.ts` file.
+
+## null and undefined
+
+Use `undefined`, do not use `null`.