Where could I find some documentation?
You may also want to have a look at the AppIndicator Reference Manual.
Ok so I've found this site:
I'm not sure how I haven't been able to find it before.
I spent some time figuring this out as well, with Python, PyGObject and GTK3. I've made a minimal working example that could also help you to get started: https://bitbucket.org/cpbotha/indicator-cpuspeed/src