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.

IMHO, todos are okay in developer or design docs, but should be avoided in user docs.  I know that has not been followed completely.   How about an issue instead, or just keep on your personal todo list.