Document the Generic RP2040 option (#546)

Fixes #522

docs/ide.rst

@@ -34,3 +34,40 @@ Debug messages from `printf` and the Core can be printed to a Serial port
 to allow for easier debugging.  Select the desired port and verbosity.
 Selecting a port for debug output does not stop a sketch from using it
 for normal operations.
+
+Generic RP2040 Support
+----------------------
+If your RP2040 board isn't in the menus you can still use it with the
+IDE bu using the `Board->Generic RP2040` menu option.  You will need to
+then set the flash size (see above) and tell the IDE how to communicate
+with the flash chip using the `Tools->Boot Stage 2` menu.
+
+Boot Stage 2 Options for Generic RP2040
+---------------------------------------
+The Arduino Pico needs to set up its internal flash interface to talk to
+whatever flash chip is in the system.  While all flash chips support a
+basic (and slow) 1-bit operation using common timings, each different
+brand (and sometimes model) of flash chip require custom timings to work
+in QSPI (4-bit) mode.  The `Boot Stage 2` menu lets you select from
+the supported timings.
+
+The options with `/2` in them divide the system clock by 2 to drive the
+bus.  Options with `/4` divide the clock by 4 and so are slower but more
+compatible.
+
+If you can't match a chip name in the menu to your flash chip, a simple
+test can be run to determine which is correct.  Simpily load the `Blink`
+example, select the first option in the `Boot Stage 2` menu, and upload.
+If that works, note it and continue.  Iterate through the options and
+note which ones work.  If an option doesn't work, unplug the chip and
+hold the BOOTSEL button down while re-inserting it to enter the ROM
+uploader mode.  (The CPU and flash will not be harmed if the test fails.)
+
+If one of the custom bootloaders (not `Generic SPI /2 or /4`) worked, use
+that option to get best performance.  If none worked other than the
+`Generic SPI /2 or /4` then use that.  The `/2` options of all models
+is preferred as it is faster, but some boards do require `/4` on the
+custom chip interfaces.
+
+When in doubt, `Generic SPI /4` should work with any flash chip but is
+slow.