Update CONTRIBUTING.md

Author: dplewis

CONTRIBUTING.md

@@ -21,6 +21,7 @@
   - [Good to Know](#good-to-know)
   - [Troubleshooting](#troubleshooting)
   - [Please Do's](#please-dos)
+  - [TypeScript Tests](#typescript-tests)
   - [Test against Postgres](#test-against-postgres)
     - [Postgres with Docker](#postgres-with-docker)
 - [Breaking Changes](#breaking-changes)
@@ -239,6 +240,14 @@ Once you have babel running in watch mode, you can start making changes to parse
 * Mocks belong in the `spec/support` folder.
 * Please consider if any changes to the [docs](http://docs.parseplatform.org) are needed or add additional sections in the case of an enhancement or feature.
 
+#### TypeScript Tests
+
+Type tests are located in [/types/tests.ts](/types/tests.ts) and are responsible for ensure types generated for each class is behaving as expected. Types must be generated using `npm run build:types` and should not be manually changed. These types are `.d.ts` files located in [/types](/types).
+
+When developing type definitions you can use `npm run watch:ts` in order to rebuild your changes automatically upon each save.
+
+Use `npm run test:types` in order to run types tests against generated `.d.ts` files.
+
 ### Test against Postgres
 
 If your pull request introduces a change that may affect the storage or retrieval of objects, you may want to make sure it plays nice with Postgres.