Add a document about node.

Add an initial node documentation.   The json example uses new api convention (#2478), which will hopefully be merged soon if no one objects.

ddysher is non-native english speaker, feel free to point out any grammar error.

Hmm... yeah, I put a note here because I don't want user to be confused when apiserver refuse the creation.  We need to handle this case properly.

I'll add this to my own TODO list, and just mention the case here.