o Point out that this old board does not support 64x64 out of the box,

   but that there is a hardware hack around as mentioned in Issue #360

adapter/passive-3/README.md

@@ -1,13 +1,19 @@
 Adapter PCB to support up to 3 panel chains
 ===========================================
 
-   * Passive board. Simple, but might need to define `--led-slowdown-gpio` if you see
-     glitches. Consider using the [active board](../active-3) in that case.
+   * This is a passive board. It is simple, but the logic level will be out of
+     spec for the LED matrix (3.3V vs. 5V) which might or might not work.
+     Driving long cables with the GPIO pins is also not a good idea.
+   * You typically want to consider using the [active board](../active-3).
+   * Works for Matrix up to 1:16 multiplexing (32 rows). For 1:32 multiplexing,
+     you want to use the [active board](../active-3). You can of also hack
+     this board as [suggested in this bugtracker entry](https://github.com/hzeller/rpi-rgb-led-matrix/issues/360#issuecomment-321104348) to make it work with
+     64x64 boards.
    * Only really advisable, if the LED panels have 74HCT245 (as opposed to just
      74HC245) in their input stage, because then they can deal properly with
      the 3.3V logic levels coming from the Pi.
    * Supports up to three panel chains for newer plus models and
-     Raspberry Pi 2 that have 40 GPIO pins.
+     Raspberry Pi 2/3 that have 40 GPIO pins.
    * Open source KiCAD PCB EDA format.
    * (not very pretty layout, was just lazy and let the auto-router do it).
    * The FAB files are provided as [passive3-rpi-hub75-adapter-fab.zip](./passive3-rpi-hub75-adapter-fab.zip)