Added documentation. Added keep-alive cheking to thermo_sensor

readme.md

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+System of monitoring and controlling heating.
+
+Modules (called units) send their data via UDP.
+Each datagram contains JSON object of following form:
+{
+	"id": "<ID>",
+	"ip": "<IP>",
+	"v1": "123",
+	...
+}
+
+Where ID is unit's identifier (MAC address usually),
+IP is unit's current IP address (for server to be able
+to contact it).
+Other values in object are names and values of sensors 
+available on the unit.
+
+Server could send datagram with some commands to unit.
+For simplicity datagram is simple string with command and
+optional arguments, everything separated with character 0x01.
+
+Possible commands are:
+
+1. ping - no arguments. Informs unit that server is still alive
+and able to communicate