software/demo: add short README.

litex/soc/software/demo/README.md

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+[> Bare Metal Demo App
+----------------------
+
+This directory provides a minimal bare metal demo app that demonstrates how to easily create a bare metal C application and load it/run it on the CPU of a SoC.
+
+[> Build
+--------
+
+Imagine you just build Arty example design from LiteX-Boards, to build the demo app, run in this directory:
+ `$ export BUILD_DIR=xxyy/litex/litex/boards/targets/build/arty && make`
+
+[> Load
+-------
+
+Loading the compiled demo app can be done in different ways as explain in LiteX's wiki:
+https://github.com/enjoy-digital/litex/wiki/Load-Application-Code-To-CPU
+
+Since our app is small and for simplicity we'll just load it over serial here:
+ `$ lxterm /dev/ttyUSBX --kernel=demo.bin`
+
+You should see the minimal demo app running and should be able to interact with it:
+
+    --============== Boot ==================--
+    Booting from serial...
+    Press Q or ESC to abort boot completely.
+    sL5DdSMmkekro
+    [LXTERM] Received firmware download request from the device.
+    [LXTERM] Uploading demo.bin to 0x40000000 (9264 bytes)...
+    [LXTERM] Upload complete (9.8KB/s).
+    [LXTERM] Booting the device.
+    [LXTERM] Done.
+    Executing booted program at 0x40000000
+
+    --============= Liftoff! ===============--
+
+    LiteX minimal demo app built Dec 10 2020 17:13:02
+
+    Available commands:
+    help               - Show this command
+    reboot             - Reboot CPU
+    led                - Led demo
+    donut              - Spinning Donut demo
+    litex-demo-app> led
+    Led demo...
+    Counter mode...
+    Shift mode...
+    Dance mode...
+    litex-demo-app> donut
+    Donut demo...
+
+                                          $$$$$@@@@@
+                                      $##########$$$$$$$$
+                                   ###*!!!!!!!!!***##$$$$$$
+                                 ***!!====;;;;===!!**###$$$$#
+                                **!===;;;:::::;:===!!**####$##
+                              !*!!==;;:~-,,.,-~::;;=!!**#######!
+                              !!!!=;:~-,.......-~::==!!***#####*
+                             !!!!==;~~-.........,-:;==!!***###**!
+                             !**!!=;:~-...     ..-:;=!!!********!
+                            ;!*#####*!!;.       ~:;==!!!******!!=
+                            :!*###$$$$#*!      :;==!!!!!****!!!=;
+                            ~=!*#$$$@@@$$##!!!!!!!!!!!!****!!!!=;
+                             ;=!*#$$$@@@@$$#*******!*!!*!!!!!==;~
+                             -;!*###$$$$$$$###******!!!!!!!===;~
+                              -;!!*####$#####******!!!!!!==;;:-
+                               ,:=!!!!**#**#***!!!!!!!====;:~,
+                                 -:==!!!*!!*!!!!!!!===;;;:~-
+                                   .~:;;========;=;;:::~-,
+                                      .--~~::::~:~~--,.
+    litex-demo-app>
+
+[> Going further
+----------------
+To create more complex apps, feel free to explore the source code of the BIOS or other open source projects build with LiteX at https://github.com/enjoy-digital/litex/wiki/Projects.