Archive and update README to indicate migration to monorepo

[Dano Morrison]

[Kyle Kelley]

Interestingly enough this one isn't even in the monorepo, largely because it became part of @nteract/messaging. We should probably change the README to just state that the repo was more of a "motivation piece" for aligning sockets on RxJS. It's a bit out of date now since we multiplex all the channels to match that of the jupyter web server. That's had some nice properties since we can case of which channel a message should be routed to as part of the RxJS streams.