Add usage example for adding an API endpoint

[Kaptard]

Not sure yet which feature works best with this, but the following features should be covered:

• Dynamic URL & filepath-based URL
• Query values and defaults
• Authorization scope
• Per-Endpoint rate limiting
• Caching
• Pub/Sub model

Required for #49 (link)

Edit: Might be a good idea to tie this with the notification example, so people have a transition between the two and we can explain the missing gaps that pop up when only focusing on the UI or API part.