Add Interface Specification for data output (#827)

DATA_FORMAT.md

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+# JSON Data fields
+
+## Message Data
+These fields are the primary data fields containing the most basic message data and used to identify the specific device. 
+For some devices these are the *only* fields contained in the message, as the message itself consitutes an event from this
+particular device model.
+
+* **time** (string) (Required)
+  * Time stamp. String containing date and time of when the message was received. Format is dependent on
+    current locale unless the `-U` command line argument is used.
+
+* **model** (string) (Required)
+  * Device model. Human readable string concisely describing the device by manufacturer name 
+    and manufacturers model designation according to the following syntax: `"<Manufacturer>-<Model>"`. 
+  * It is common for devices to be sold under different brands, however the Original Equipment Manufacturer name
+    shall be used, where possible to identify. 
+  * Avoid redundant word like "sensor", "wireless" etc. unless it is part of the manufacturers model designation. 
+  * Avoid adding device type designations like "Switch", "Temperature", "Thermostat", "Weather Station" etc. Device type can 
+    be inferred from the data content. 
+  * Avoid the special characters: `"/&$*#+[]()"`.
+  * Length of *model* string should be less than 32 characters.
+
+* **id** (string) (Optional)
+  * Device identification. Used to differentiate between devices of same *model*. 
+    Depending of device model it may be a non-volatile value programmed into the device, 
+    a volatile value that changes at each power on (or battery change), or a value configurable by
+    user e.g. by switch or jumpers. No assumptions should be made to the id value other than it contains
+    a unique sequence of alphanumeric characters.
+  * Length of *id* should be less than 16 characters.
+
+* **channel** (string) (Optional)
+  * Secondary device identification. For devices with more than one identification value 
+    (e.g. both an internal value and a switch).
+
+* **mic** (string) (Optional)
+  * Message integrity check. String describing the method used for ensuring the data integrity
+    of the message. Protocol decoders for devices without mic will be disabled by default as 
+    they are prone to excessive false positives.
+  * Possible values:
+    * "CRC" - Cyclic Redundancy Check.
+    * "CHECKSUM" - Accumulated sum of data.
+    * "PARITY" - Parity bit (odd, even, multiple)
+
+## Common Device Data
+Various data fields, which are common across devices of different types 
+
+* **battery** (double) (Optional)
+  * Battery status indication as a level between 0 (empty) and 1 (full). If the sensor can only report a binary status the value shall be 1 for "OK" and 0 for "LOW".
+
+* **battery_V** (double) (Optional)
+  * Battery level in Volts. Should be suppleanted by *battery* status indication if possible.
+
+## Sensor Data
+Due to the large variance in sensor types this list of common values is non-exhaustive. Additional data value fields should follow the form: `<Type>_<Unit>`, where *Unit* should be in sensors native units insofar possible with no conversion.
+Examples:
+
+* **temperature_C** (**temperature_F**) (double) (Optional)
+  * Temperature from a temperature sensor in degrees Celcius (Fahrenheit).
+  
+* **setpoint_C** (**setpoint_F**) (double) (Optional)
+  * Thermal set point of a thermostat device in degrees Celcius (Fahrenheit).
+  
+* **humidity** (double) (Optional)
+  * Humidity from a hygrometer sensor in % relative humidity
+  
+* **wind_dir_deg** (double) (Optional)
+  * Wind direction from wind sensor in compass direction degrees.
+  
+* **wind_speed_mps** (double) (Optional)
+  * Average wind speed from wind sensor in m/s. Averaging time is sensor dependant.
+
+* **gust_speed_mps** (double) (Optional)
+  * Gust wind speed from wind sensor in m/s. 
+
+* **rain_mm** (**rain_in**) (double) (Optional)
+  * Rainfall from rain sensor in mm (inches) since last reset. Reset method is device dependant.
+  
+* **rain_rate_mmph** (**rain_rate_inph**) (double) (Optional)
+  * Rainfall rate from rain sensor in mm per hour (inches per hour).
+  
+* **pressure_hPa** (**pressure_psi**) (double) (Optional)
+  * Air pressure from barometer or Tire Pressure Monitor in hPa (psi)
+