Document the MQTT encryption

Readme.md

@@ -75,7 +75,7 @@ Tracking devices
  * set distance bewteen data points: send no data, when distance is below a certain limit (e.g. 10m)
  * activate "stop recording, if no movement is detected"
 * Owntracks client APP
- * The app sends the position data to a MQTT server (can be encrypted). Topic: `owntracks/<username>`
+ * The app sends the position data to a MQTT server (can be encrypted). Topic: `owntracks/<username>` e.g. `owntracks/gps`
  * A bridge code, which extracts the data from the MQTT server and sends them to the Simple GPS Data Server, is available under `mqtt-bridge`
  * The example includes the MQTT server as well
  * This is a pure docker implementation

mqtt-bridge/Readme.md

@@ -1,9 +1,12 @@
-Run a MQTT server and Bridge to the Simple GPS-Data Server
-==========================================================
+# Run a MQTT server and Bridge to the Simple GPS-Data Server
 
 Projects like Owntracks (e.g. APP) send the GPS data to a MQTT server. In order to get these data stored and visualized, a bridge code subscribes to the data and sends the data to the TCP-Bridge.
 
 This dockerized version starts a Mosquitto server and runs the bridge code (Golang program) in a separate container.
 
-The configuration is done in the `docker-compose.yml` file by changing the environment variables.
-
+The MQTT configuration is done in the corresponding `config` directory.
+Parameters are configurated in`docker-compose.yml` by changing the environment variables.
+
+## Encrypting the payload
+The Owntracks client allows to encrypt the payload, e.g. the GPS data. This requires to enter a pre-shared key in the client. Only a single key can used for all client devices. 
+The PSK has to be known by the bridge code and has to be entered as a parameter in `docker-compose.yml` (`PSK_ENC`).