add the readme for the lib

picosim/libs/stdio_msc_usb/README.txt

@@ -0,0 +1,23 @@
+This library allows USB mass storage access together with USB CDC
+stdio. Intended to be used together with
+https://github.com/carlk3/no-OS-FatFS-SD-SDIO-SPI-RPi-Pico .
+
+Based on pico-sdk/src/rp2_common/pico_stdio_usb and
+pico-sdk/lib/tinyusb/examples/device/cdc_msc .
+
+See CMakeLists.txt, picosim.c, and simcfg.c in srcsim to see how to
+use this.
+
+Basically:
+
+Add with "add_subdirectory" and "target_link_libraries(tinyusb_device
+stdio_msc_usb)" to CMakeLists.txt. Remove "pico_enable_stdio_usb".
+
+Call "tusb_init()" and "stdio_msc_usb_init()" in "main()". Activate
+the mass storage access with "stdio_msc_usb_do_msc()". This will wait
+until the medium is ejected by the host OS.
+
+This is done this way, because the FatFS library needs exclusive
+access to the storage medium. So USB access can only be activated in
+the configuration menu to allow copying of files and disk images
+to/from the host OS.