Docs improvement: State returned from getInitialState(input) should be based on the input

[maberer]

We should document, that "State returned from getInitialState(input) should be based on the input". Otherwise, hard coded objects do not only cause a re-render (in case of an empty array), they even get reseted to their initial state (when they are changed in the meantime).

As a consequence, there is nothing like "truly private properties", that keep their state when the component gets re-rendered because of a changed input property...

The issue is discussed here: #100