README: Provide basic instructions on how to set up MicroPython.

Signed-off-by: Damien George <[email protected]>

Author: dpgeorge

README.md

@@ -2,3 +2,15 @@ MicroPython custom boards
 =========================
 
 This repository shows how to define custom MicroPython boards.
+
+To set up MicroPython, check out the submodule and build `mpy-cross`:
+
+    $ git submodule update --init lib/micropython
+    $ make -C lib/micropython/mpy-cross
+
+The various custom boards will require certain submodules of MicroPython
+itself.  These can be initialised via:
+
+    $ cd lib/micropython
+    $ git submodule update --init lib/stm32lib lib/berkeley-db-1.xx
+    $ cd ../..