Improved custom data points docs

documentation/developer-guide.md

@@ -1,5 +1,43 @@
 
 ## Tips if you want to modify the code
+### Adding data points to the returned readings
+#### Simple data mangling
+You can customise the sensor readings to be saved and uploaded by adjusting the "reading" dictionary; Adding extra information or removing data points that you don't want. Make any adustments after the line populating the reading dictionary:
+```
+reading = enviro.get_sensor_readings()
+```
+
+For example:
+```
+reading = enviro.get_sensor_readings()
+
+del reading["temperature"]        # remove the temperature data
+reading["custom"] = my_reading()  # add my custom reading value
+```
+
+#### Custom data points (BME688 example)
+Add simple built in module calls in main.py after the reading dictionary is populated and modify the reading dictionary as required
+
+Add your code after the line:
+```
+reading = enviro.get_sensor_readings()
+```
+
+A simple BME688 module example:
+```
+reading = enviro.get_sensor_readings()
+
+from breakout_bme68x import BreakoutBME68X
+bme = BreakoutBME68X(enviro.i2c)
+temperature, pressure, humidity, gas_resistance, status, gas_index, meas_index = bme.read()
+reading["temperature2"] = temperature
+```
+Credit: @hfg-gmuend in [#178](https://github.com/pimoroni/enviro/issues/178)
+
+The above code will overwrite the returned data if you use the same key name e.g. "temperature", ensure this is what you want to do, or otherwise pick a unique name for your new data point e.g. "temperature2"
+
+#### Modifying specific board sensor collections
+If the existing readings from a specific board require adjustment, for example adding a sea level adjusted value for atmospheric pressure readings. This should be done in the in board specific file in the boards directory, modifying the necessary lines in the get_sensor_readings() function.
 
 ### Code structure
 

main.py

@@ -68,14 +68,15 @@
   # TODO should the board auto take a reading when the timer has been set, or wait for the time?
   # take a reading from the onboard sensors
   enviro.logging.debug(f"> taking new reading")
+
+  # Take a reading from the configured boards sensors, returns a dictionary of
+  # reading name and value pairs
+  # e.g. reading = {"temperature" : 19.1, "humidity" : 64,...}
   reading = enviro.get_sensor_readings()
 
-  # here you can customise the sensor readings by adding extra information
-  # or removing readings that you don't want, for example:
-  # 
-  #   del readings["temperature"]        # remove the temperature reading
-  #
-  #   readings["custom"] = my_reading()  # add my custom reading value
+  # Here you can customise the returned date, adding or removing data points
+  # Refer to the documentation for more information: 
+  # https://github.com/pimoroni/enviro/blob/main/documentation/developer-guide.md
 
   # is an upload destination set?
   if enviro.config.destination: