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:

• https://github.com/libp2p/specs/pull/568
• https://github.com/multiformats/multiaddr/issues/165

closes #165

Before Merge

• [ ] Allow at least 24 hours for community input
• [x] 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.