Added a link to pico-littlefs-usb (FAT12 emulation)

Implemented by oyama, pico-littlefs-usb provides an easy interface to
littlefs by emulating a FAT12 filesystem over USB.

There are some tradeoffs to this, but being able to mount a littlefs
device without installing additional drivers is very nice. Maybe in the
future devices could provide both a FAT and raw endpoint for
easy/advanced filesystem access.

README.md

@@ -237,6 +237,10 @@ License Identifiers that are here available: http://spdx.org/licenses/
 - [mklfs] - A command line tool built by the [Lua RTOS] guys for making
   littlefs images from a host PC. Supports Windows, Mac OS, and Linux.
 
+- [pico-littlefs-usb] - An interface for littlefs that emulates a FAT12
+  filesystem over USB. Allows mounting littlefs on a host PC without additional
+  drivers.
+
 - [Mbed OS] - The easiest way to get started with littlefs is to jump into Mbed
   which already has block device drivers for most forms of embedded storage.
   littlefs is available in Mbed OS as the [LittleFileSystem] class.
@@ -278,3 +282,4 @@ License Identifiers that are here available: http://spdx.org/licenses/
 [littlefs2-rust]: https://crates.io/crates/littlefs2
 [chamelon]: https://github.com/yomimono/chamelon
 [nim-littlefs]: https://github.com/Graveflo/nim-littlefs
+[pico-littlefs-usb]: https://github.com/oyama/pico-littlefs-usb