Add description of bluetooth multiplexing protocol

Summary:
As mentioned on telegram, there are some fundamental issues with bluetooth. The only approach I could get working was multiplexing: manually sending multiple streams of data over a single connection.

This is the description of that multiplexing protocol. I have an implementation of this protocol, but I'd first like some feedback to see if this is what we're going to use before finishing up those patches.

See the document itself for more details. I make the protocol forward-compatible, although I don't expect we will need ever that.

Test Plan: None, this is just a description.

Reviewers: KDE Connect, andyholmes, albertvaka

Reviewed By: KDE Connect, albertvaka

Subscribers: albertvaka, kdeconnect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D17987