discv5: add intro document, handshake, more theory

[fjl]

This is my second pass over the spec after working on the implementation for a while. Changes include:

• There is now a high-level intro document that ties the specs together.
• The 'topics' document is now 'theory' and contains a description of the routing table, lookup etc. in addition to topic advertisement content.
• Wire protocol is updated based on feedback from implementation.
  • The new handshake replaces the WHOAREYOU and IAM messages
  • conversation-nonce is removed in favor of a simple request-id
  • NODES replaces NEIGHBORS and TOPICNODES
  • message data lists are now up to date with reality
  • All the 'maybes' and 'proposals' are removed from the spec text.
• The 'requirements' document is now 'rationale' but is otherwise unchanged.

Closes #60

[fjl]

Thank you for the review. Most of your comments are addressed now, but I'm not going to add content about alternative transports to this spec. I will add a section to the rationale about UDP to address your points.