Polina Goltsman issue 9

"   Node-ID:  A value of fixed but configurable length that uniquely
      identifies a node.  Node-IDs of all 0s and all 1s are reserved and
      are invalid Node-IDs.  A value of zero is not used in the wire
      protocol but can be used to indicate an invalid node in
      implementations and APIs.  The Node-ID of all 1s is used on the
      wire protocol as a wildcard.

what means "invalid" exactly? A wildcard can be used at least on
the wire so it is not invalid being used as destination Node-IDs.
So the above definition is slightly contradictory."

- Restructured the paragraph.