sdcardio: add a note about sharing the SPI bus with SD cards and other devices

Similar to adafruit/Adafruit_CircuitPython_SD#44

Author: jepler

shared-bindings/sdcardio/SDCard.c

@@ -55,6 +55,12 @@
 //|         Data transfers use the specified baurate (rounded down to one that is supported by
 //|         the microcontroller)
 //|
+//|         .. important::
+//|            If the same SPI bus is shared with other peripherals, it is important that
+//|            the SD card be initialized before accessing any other peripheral on the bus.
+//|            Failure to do so can prevent the SD card from being recognized until it is
+//|            powered off or re-inserted.
+//|
 //|         Example usage:
 //|
 //|         .. code-block:: python