Update vehicle position documentation

docs/vehicle-positions.md

@@ -1,5 +1,7 @@
 ### Vehicle positions
 
+#### Overview
+
 The vehicle positions are sent to Digitransit through a chain of services which are drawn in the following
 diagram:
 
@@ -9,12 +11,32 @@ Originally, the data is provided by VVS and picked up by a GTFS-RT-to-MQTT bridg
 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
+Digitransit can connect to the MQTT broker via Websockets and subscribes 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
-```
+```
+
+#### Ansible
+
+The code for the vehicle positions is in the `leonardehrenfried/baseline` repository: https://github.com/leonardehrenfried/ansible-baseline/tree/main/roles/vehicle_positions
+
+#### Configuration
+
+The following ansible configuration variables are available:
+
+- `vehicle_positions_poll_interval`: how often the source GTFS-RT feeds is polled, in seconds (default: 5)
+
+#### Ports
+
+Generally, the only port that is really necessary is 443 as this is the one to which Digitransit
+connects to open the secure websocket connection. (It's very common to run the MQTT over the websocket
+protocol.)
+
+However, if you want to use the native MQTT protocol, you can also connect using port 1883. In order
+to do this you need to open the port on the server first, which is done with the ansible configuration 
+variable `firewall_allowed_tcp_ports`.