Add link to PCB heater mod (#966)

I made a PCB for the heater for those that prefer making a new PCB rather than modifying the existing one.

src/docs/devices/MQ-7/index.md

@@ -12,7 +12,7 @@ The MQ-7 sensor can be used for detecting carbon monoxide (CO) gas. This is done
 
 ## Heater Mod
 
-This will remove the digital output of the sensor and replace it with a digital input to control the heater of the MQ-7
+This will remove the digital output of the sensor and replace it with a digital input to control the heater of the MQ-7. There is also an OSS PCB design available under https://github.com/fablab-paderborn/mq7-adapter-board, which has the same circuit as the module after the heater mod.
 
 ### What you need