The documentation on get_json has been wrong ever since the very first commit
to synapse...

Also:
 - change the REST endpoint to have a "S" on the end (so it's now /read_markers)
 - change the content of the m.read_up_to event to have the key "event_id" instead of "marker".

Update README.rst

The FreeBSD port has been moved to the net-im category

Implement Read Marker API

Reduce federation replication traffic

Add a counter metric for successfully-sent transactions

Dedupe KeyedEdu and Devices federation repl traffic

Remove HTTP replication APIs

Fix getting latest device IP for user with no devices

 - This change causes a 405 to be sent if "m.read_marker" is set via /account_data
 - This also fixes-up the RM endpoint so that it actually Works.

(See https://docs.google.com/document/d/1UWqdS-e1sdwkLDUY0wA4gZyIkRp-ekjsLZ8k6g_Zvso/edit#heading=h.lndohpg8at5u)