ADDDING_A_MODULE.md

Adding a Module

1. Make the module's folder in this project's root directory. Convention is to use lowercase.
2. Make an entry in modules.json. See the example module there for more info.
3. Allow that repo access to the Organisation Secrets APP_ID and APP_KEY.
4. Add an event handler in the module's git repository. See .github/workflows/exampleEventSender.yml.disabled for info.