URI encoding

[MarcoPolo]

Summary

Adds steps to describe how a Multiaddr becomes a URI. Useful for contexts that expect a URI.

Prior discussions and context:

• libp2phttp: Define the multiaddr URI libp2p/specs#568
• Multiaddr URI scheme #165

closes #165

Before Merge

• Allow at least 24 hours for community input
• If this is a new protocol, has the change been applied to https://github.com/multiformats/multicodec as well? If so, please link the multicodec PR.