[wip] Web Pathing Specification: initial outline with TODOs

[lidel]

The goal of this specification is to close #432 and define a subset of possible content paths that ensures compatibility with existing HTTP and Web Platform standards, and have clear MUSTs and SHOULDs that we can use when discussing implementation details of projects like ipfs-chromium's Intent to Prototype: Verifying IPFS client.

Pushing an extremely early draft of the scope to get early feedback.

Everyone is invited to comment on the PR, focusing on TODOs, MUSTs and SHOULDs and suggest improvements, especially if something is missing :pray: