High level documentation providing overview of ontology

[mattyg]

For developers working on tx5, it would be helpful to have some docs that give an overview of the various components and how they fit together. I believe this are all the components and their relationships:

• Endpoint
  • Sig
    • Hub
  • set of Peer
    • FramedConn
      • Conn
        • Webrtc

AC:

• Review components, add module-level comments to any missing them
• Add an overview module-level comment to each crate, outlining the components within it.