Update README.md

Author: rorosaurus

README.md

@@ -4,7 +4,9 @@ Inspired by Adafruit's [RGB Matrix Featherwing Kit](https://www.adafruit.com/pro
 
 This shield only works with [SmartMatrix](https://github.com/pixelmatix/SmartMatrix/tree/teensylc), not [PxMatrix](https://github.com/2dom/PxMatrix).
 
-If you want to work with PxMatrix, I highly recommend checking out [Brian Lough's shields](https://www.tindie.com/stores/brianlough/)! They are designed much better than mine! I simply prefer SmartMatrix, which is why I designed this shield.
+If you want to work with PxMatrix, I highly recommend checking out [Brian Lough's shields](https://www.tindie.com/stores/brianlough/)! They are designed much better than mine!
+
+I simply prefer SmartMatrix, which is why I designed this shield. There are [some shields in the SmartMatrix repo](https://github.com/pixelmatix/SmartMatrix/tree/teensylc/extras/hardware) but these appear to be active and require extra components. My shield is passive and just connects the pins to the right place. It lacks a proper level-shifter, so it assumes your panel will accept the 3.3V logic from the ESP32. It lacks an external latch, which eats up a few more pins from your ESP32.
 
 ## Features
 * Connects all necessary pins to drive HUB75 panels using `ESP32_FORUM_PINOUT` from [MatrixHardware_ESP32_V0.h](https://github.com/pixelmatix/SmartMatrix/blob/teensylc/src/MatrixHardware_ESP32_V0.h)