Add API documentation

[emilydaemon]

from what I can tell, the API is not mentioned anywhere, not on the front page, not on the README.md, nothing I've gotten an idea for a RiiTag homebrew app that could display your RiiTag without having to use the Internet Channel, but since there is no documentation, I have to look at the source code of this repository and RiiTag-RPC. If nobody here's up to do it, and the API turns out to be simple enough for me to explain, I could probably try to write some. Thank you!