pcb/main board: add errata

pcb/MainBoard/README.md

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+# Main Board
+
+50x50mm microcontroller module, driving LedArrayMux modules. Contains an RP2040 microcontroller, a CAN transceiver, PWM audio output, current sensing, and level shifters to work around LedArrayMux bugs.
+
+## Revisions
+
+### Rev A
+
+Errata:
+
+1. Level shifters are too slow to be useful
+  - Workaround: decrease pullup values to obnoxiously low ones (100-180Ω); do not use level shifters; increase PIO delays until functional
+2. CAN receive signal is not level shifted, which exposes RP2040 to a 5V signal
+  - Workaround: Do nothing; replace with a 3v3 CAN transceiver
+3. The 431 shunt voltage reference does not work correctly
+  - Workaround: use main 3v3 regulator for vref
+4. PWM audio signal filter is suspected to be responsible for poor quality audio