Add small spec on addressing

[yusefnapora]

Reading through the specs, I realized that most of them assume that you're familiar with multiaddr. The multiaddr README doesn't go into much detail on how it's used in libp2p, and I think the specs would benefit from an overview of how addresses work in libp2p.

Main points to cover:

• libp2p uses multiaddr to encode transport addresses
  • briefly describe string and binary formats
• multiaddrs are composable (describe encapsulation / decapsulation)
  • show examples of e.g. websockets over TCP/IP
• examples of addresses for all currently supported transports
• note about extending multiaddr to support new protocols (e.g. p2p-circuit)
  • should mention that it's rarely needed, but possible if you have the right use case