examples/filesystem: Update and add to README

Reflect the updated behavior of the filesystem example and added more examples on the usage on real boards.

Remove the remarks about mtd and MTD_0.

Co-Authored-By: benpicco <benpicco@googlemail.com>

examples/filesystem/README.md

@@ -7,16 +7,19 @@ application.
 
 In particular, this example shows:
 
-- how to mount/format/unmount a file system, either with spiffs, littlefs, fatfs
-  or constfs
+- how to mount/format/unmount a file system, either with constfs for all
+  devices and the default fs for devices that have a filesystem mountpoint
+  configured by the board.
 - how to open/read/write/close a file with and without newlib
 
-In RIOT, most file systems use a `mtd` as flash interface. So to use this
-example one must define `MTD_0`. `MTD_0` is a pointer to a `mtd_dev_t`
-instance.
+A `constfs` file system is demonstrated with two files for all targets
+to demonstrate the file system functionality without an external mass
+storage device.
+
+For targets that feature an onboard external mass storage device with a
+mount point configured, like the Nordic nRF52840 Development Kit
+(see `boards/nrf52840dk`), the flash chip is automatically added by VFS.
 
-This example uses `littlefs` as default file system on the whole `mtd`.
-A `constfs` file system is also demonstrated with two files.
 
 All the RIOT file systems are used through the `vfs` interface, and on most
 platforms files can be accessed transparently with `open/close/read/write/...`
@@ -75,3 +78,52 @@ Hello World!
 cat /const/hello-riot
 Hello RIOT!
 ```
+
+## Example on `nrf52840dk` with `littlefs`
+
+- Build and flash `filesystem` example application of the `nrf52840dk` target
+
+```
+make BOARD=nrf52840dk -C examples/filesystem all flash
+[...]
+```
+
+ - When connecting to the board via the serial console, it should display the
+   following output:
+```
+main(): This is RIOT! (Version: 2024.04-devel-254-gee0f6d)
+constfs mounted successfully
+>
+```
+
+- The flash chip is empty by default and has to be formatted, which can be
+  done with the `vfs format /nvm0` command. After that it has to be mounted
+  with `vfs mount /nvm0`. Once the flash is formatted, it will be
+  automatically mounted during boot.
+```
+> vfs format /nvm0
+> vfs mount /nvm0
+> vfs df
+Mountpoint              Total         Used    Available     Use%
+/const                   27 B         27 B          0 B     100%
+/nvm0                   8 MiB        8 KiB     8184 KiB       0%
+```
+
+- After formatting, the flash is empty, but new files can be created
+  with `tee` and printed with `cat`.
+
+```
+> ls /nvm0
+./
+../
+total 0 files
+>
+> tee /nvm0/testfile "This is written to the flash!"
+> cat /nvm0/testfile
+This is written to the flash!>
+> vfs ls /nvm0
+./
+../
+testfile	30 B
+total 1 files
+```