docs: instructions for locally building typings

Author: haganbmj

.github/CONTRIBUTING.md

@@ -7,7 +7,11 @@
 - Install dependencies with `npm install`
 - Generate the browser distributable Javascript file with `npm run build`  
   - Or `npm run watch` to automatically build when files are modified.
-- This project makes use of eslint, run `npm test` for linting and basic unit testing.
+- This project makes use of eslint, run `npm run test:static` for linting and `npm run test:ava` for basic unit testing. If you have the types compiled you can also run `npm run test:types`.
+
+## Building Typings Locally
+- Export GH_TOKEN as an environment variable, using a generated Personal Access Token.
+- `npm run build:types`
 
 ## Contributing Guidelines
 - Please use descriptive commit messages.