To contribute, you'll need to install retype. To do it via NPM:
npm install retypeapp
That's all you need.
To run it:
retype watch
NOTE: You may need to go into Command Prompt/Terminal as an administrator and run Set-ExecutionPolicy RemoteSigned for retype watch to work; it allows you to run scripts on your machine.
After that, it'll load on http://localhost:5000/ as default. You can refer to the Retype docs to get a better idea of how to add to the site. It's all very easy.