Add basic instructions to README.

README.md

-This is the Micro Python project
+The Micro Python project
+========================
+
+Subdirectories:
+- py/ -- the core Python implementation, including compiler and runtime.
+- unix/ -- a version of Micro Python that runs on Unix.
+- stm/ -- a version of Micro Python that runs on the Micro Python board
+  with an STM32F405RG.
+
+"make" is used to build the components.
+
+The unix version
+----------------
+
+The "unix" part requires a standard Unix environment with gcc.  It works
+only for 64-bit machines due to a small piece of x86-64 assembler for
+the exception handling.
+
+To build:
+    $ cd unix
+    $ make
+
+Then to test it:
+    $ ./py
+    >>> list(5 * x + y for x in range(10) for y in [4, 2, 1])
+
+The stm version
+---------------
+
+The "stm" part requires an ARM compiler, arm-none-eabi-gcc, and associated
+bin-utils.  For those using Arch Linux, you need arm-none-eabi-binutils and
+arm-none-eabi-gcc packages from the AUR.  Otherwise, try here:
+https://launchpad.net/gcc-arm-embedded
+
+To build:
+    $ cd stm
+    $ make
+
+Then to flash it via USB DFU to your device:
+    $ dfu-util -a 0 -D build/flash.dfu
+
+You will need the dfu-util program, on Arch Linux it's dfu-util-git in the AUR.