Added initial version of API design specification

Not yet complete, first doc für review purposes. Future design specs shall be pretty similar.
Not sure this is quite a meaningful distinction at this level. Watch requests just return results like any other -- it's true in the background they create server-side resources which respond to future Next() calls, which are a little bit weird in that they may take a long time to return, but it's still following the websocket-request/response model.

The stranger ones are the HTTP requests like PutCharm, and I think it is worth calling those out at this level -- they're still part of the API, and they're following a substantially different model to the rest.