Add documentation about MQTT chain

docs/vehicle-positions.md

@@ -0,0 +1,20 @@
+### Vehicle positions
+
+The vehicle positions are sent to Digitransit through a chain of services which are drawn in the following
+diagram:
+
+![vehicle positions diagram](vehicle-positions.md)
+
+Originally, the data is provided by VVS and picked up by a GTFS-RT-to-MQTT bridge. This then
+publishes the data onto a MQTT broker (Mosquitto) which makes them available through a series
+of topics.
+
+Digitransit can cannot to the MQTT broker via Websockets and subscribe to the data. Depending
+on the exact poll frequency, it may take a few seconds until data really appears in the app.
+
+If you want to look at the raw MQTT topic you can do the following:
+
+```sh
+npm install -g mqtt
+mqtt subscribe -h vehiclepositions.stadtnavi.eu -p 443 -l wss -v -t "#" -i my-client
+```