Update README.md

README.md

@@ -41,9 +41,9 @@ lamp in your Home Assistant setup.
   Go ahead, make it so! :-)
  
 * **Possibilities to extend the device's functionality through hardware mods.**
-  There are GPIO pins that are not in use. If "tinkerer" is your middle name,
-  you can use those pins to come up with your own hardware hacks to extend
-  the device's functionality. 
+  There are [GPIO pins that are not in use](doc/hardware.md#esp32-pinout).
+  If "tinkerer" is your middle name, you can use those pins to come up with
+  your own hardware hacks to extend the device's functionality. 
 
 ## Quick start guide