Note incomplete pin list

Board-Nucleo-F767ZI.md

@@ -15,6 +15,10 @@ Status "not officially supported but should be fully functional" as of 2018-08.
  * USB OTG
  * Ethernet port (probably not yet supported by uPy)
 
+# Caveats
+
+The default build for the board only exposes the Arduino compatibility pins plus the "Zio" ones up to D25 (i.e. the rest are not in `pyb.Pin.cpu` nor `machine.Pin.cpu`).  Tuple-based access is documented but doesn't seem to exist.  So you'll need to edit `pins.csv`, [add the rest](https://gist.github.com/zwbrbr/f9202e3cc9288b459b8b116b83125c06) and rebuild to get access to them.
+
 # Flashing
 
 Probably requires newer OpenOCD than 0.9.0, which complained "Cannot identify target as a STM32 family" despite getting a device ID.