Samples to show inheritance

@sigmundch @jakemac53 PTAL.
Maybe this is just me, so feel free to ignore. While correct, extender/extendee gets a bit confusing for me. I'm not even sure why. Maybe because I normally think of extension as a relationship that goes up-down, but extender-extendee seems to go down-up? No idea.

An alternative here would be: `ExtendeeElement` -> `BaseElement`, `ExtenderElement`  -> `SubSelement`