docs(README): add virtual node documentation
docs(README): add virtual node documentation

Add documentation for the structure and properties of a virtual
node used by snabbdom and created by h().

paldepind/snabbdom,snabbdom/snabbdom,snabbdom/snabbdom,staltz/snabbdom,staltz/snabbdom,paldepind/snabbdom,snabbdom/snabbdom,staltz/snabbdom,snabbdom/snabbdom