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.

Oh, yeah.  Thanks.  But I think it might be better if we change it to name, this might cause some issue for users when deploying application across versions.  I'll take a look