initial draft of URI scheme CAIP, needs examples

[bumblefudge]

To-do:

• [ ] Provide illustrative examples and corner-cases
• [ ] Mention future CAIPs as well, such as 220 and 221?
• [ ] Decide if registration should point to github URL of this CAIP or to the rendered permanent URL under our control, not msft/GH's?

This would close #243 and #67