describe monitor hook

README.md

@@ -11,3 +11,21 @@ Recorder
 1. Edit `config.h`
 2. Edit `config.mk` and select features
 2. Type `make`
+
+## Monitoring
+
+In order to monitor the _recorder_, whenever an MQTT message is received, the _recorder_ will add an epoch timestamp and the last received topic a Redis key (if configured) or a file otherwise. The Redis key looks like this:
+
+```
+redis 127.0.0.1:6379> hgetall ot-recorder-monitor
+1) "time"
+2) "1439738692"
+3) "topic"
+4) "owntracks/jjolie/ipad"
+```
+
+The `monitor` file is located relative to STORE and contains a single line, the epoch timestamp at the moment of message reception and the topic separated from eachother by a single space:
+
+```
+1439738692 owntracks/jjolie/ipad
+```