Move model to client-server for now.

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+==================
+Room Join Workflow
+==================
+
+An outline of the workflows required when a user joins a room.
+
+Discovery
+=========
+
+To join a room, a user has to discover the room by some mechanism in order to
+obtain the (opaque) Room ID and a candidate list of likely home servers that
+contain it.
+
+Sending an Invitation
+---------------------
+
+The most direct way a user discovers the existence of a room is from a
+invitation from some other user who is a member of that room.
+
+The inviter's HS sets the membership status of the invitee to "invited" in the
+"m.members" state key by sending a state update PDU. The HS then broadcasts this
+PDU among the existing members in the usual way. An invitation message is also
+sent to the invited user, containing the Room ID and the PDU ID of this
+invitation state change and potentially a list of some other home servers to use
+to accept the invite. The user's client can then choose to display it in some
+way to alert the user.
+
+[[TODO(paul): At present, no API has been designed or described to actually send
+that invite to the invited user. Likely it will be some facet of the larger
+user-user API required for presence, profile management, etc...]]
+
+Directory Service
+-----------------
+
+Alternatively, the user may discover the channel via a directory service; either
+by performing a name lookup, or some kind of browse or search acitivty. However
+this is performed, the end result is that the user's home server requests the
+Room ID and candidate list from the directory service.
+
+[[TODO(paul): At present, no API has been designed or described for this
+directory service]]
+
+
+Joining
+=======
+
+Once the ID and home servers are obtained, the user can then actually join the
+room.
+
+Accepting an Invite
+-------------------
+
+If a user has received and accepted an invitation to join a room, the invitee's
+home server can now send an invite acceptance message to a chosen candidate
+server from the list given in the invitation, citing also the PDU ID of the
+invitation as "proof" of their invite. (This is required as due to late message
+propagation it could be the case that the acceptance is received before the
+invite by some servers). If this message is allowed by the candidate server, it
+generates a new PDU that updates the invitee's membership status to "joined",
+referring back to the acceptance PDU, and broadcasts that as a state change in
+the usual way. The newly-invited user is now a full member of the room, and
+state propagation proceeds as usual.
+
+Joining a Public Room
+---------------------
+
+If a user has discovered the existence of a room they wish to join but does not
+have an active invitation, they can request to join it directly by sending a
+join message to a candidate server on the list provided by the directory
+service. As this list may be out of date, the HS should be prepared to retry
+other candidates if the chosen one is no longer aware of the room, because it
+has no users as members in it.
+
+Once a candidate server that is aware of the room has been found, it can
+broadcast an update PDU to add the member into the "m.members" key setting their
+state directly to "joined" (i.e. bypassing the two-phase invite semantics),
+remembering to include the new user's HS in that list.
+
+Knocking on a Semi-Public Room
+------------------------------
+
+If a user requests to join a room but the join mode of the room is "knock", the
+join is not immediately allowed. Instead, if the user wishes to proceed, they
+can instead post a "knock" message, which informs other members of the room that
+the would-be joiner wishes to become a member and sets their membership value to
+"knocked". If any of them wish to accept this, they can then send an invitation
+in the usual way described above. Knowing that the user has already knocked and
+expressed an interest in joining, the invited user's home server should
+immediately accept that invitation on the user's behalf, and go on to join the
+room in the usual way.
+
+[[NOTE(Erik): Though this may confuse users who expect 'X has joined' to
+actually be a user initiated action, i.e. they may expect that 'X' is actually
+looking at synapse right now?]]
+
+[[NOTE(paul): Yes, a fair point maybe we should suggest HSes don't do that, and
+just offer an invite to the user as normal]]
+
+Private and Non-Existent Rooms
+------------------------------
+
+If a user requests to join a room but the room is either unknown by the home
+server receiving the request, or is known by the join mode is "invite" and the
+user has not been invited, the server must respond that the room does not exist.
+This is to prevent leaking information about the existence and identity of
+private rooms.
+
+
+Outstanding Questions
+=====================
+
+ * Do invitations or knocks time out and expire at some point? If so when? Time
+   is hard in distributed systems.