There are not many contribution guidelines yet. If, however, you are not currently familiar with Yeoman generator development you will find some helpful links.
Basic technologies to know
Write your own Yeoman generator - This article provides you with basic insights on how to write a custom Yeoman generator
Mocha getting started - To test that the Yeoman generator is working properly a testing framework named Mocha is used
Places to look for help
Yeoman API Documentation - Besides the getting started guide, there is official documentation available that covers the complete Yeoman API