Centralize maintenance of README.md

[annevk]

This requires a bit more discussion I think about what should go in the README and what should go in a local CONTRIBUTING.

Examples that differ:

• Formatting guidelines
• Referencing implementations that ought to be fully aligned, such as whatwg-url.

Many other things are extremely consistent though and ought to be the same.

[annevk]

#49 has the first step here, but it doesn't do whatwg-url correctly.

[annevk]

28bf1f1 gave URL the flexibility to handle this. We should roll that out to all specifications so they all have the same README at the top. We can continue using this issue for that.

• Streams
• HTML
• Web IDL