stmhal: Add stmhal-specific README.md with extra details for this port.

README.md

@@ -149,3 +149,4 @@ Then to flash the code via USB DFU to your device:
 This will use the included `tools/pydfu.py` script.  If flashing the firmware
 does not work it may be because you don't have the correct permissions, and
 need to use `sudo make deploy`.
+See the README.md file in the stmhal/ directory for further details.

stmhal/README.md

+MicroPython port to STM32 MCUs
+==============================
+
+This directory contains the port of MicroPython to ST's line of STM32Fxxx
+microcontrollers.  It is based on the STM32Cube HAL library and currently
+supports: STM32F401, STM32F405, STM32F411, STM32F429, STM32F746.
+
+The officially supported boards are the line of pyboards: PYBv1.0 and PYBv1.1
+(both with STM32F405), and PYBLITEv1.0 (with STM32F411).  See
+[micropython.org/pyboard](http://www.micropython.org/pyboard/) for further
+details.
+
+Other boards that are supported include ST Discovery and Nucleo boards.
+See the boards/ subdirectory, which contains the configuration files used
+to build each individual board.
+
+Build instructions
+------------------
+
+An ARM compiler is required for the build, along with the associated binary
+utilities.  The default compiler is `arm-none-eabi-gcc`, which is available for
+Arch Linux via the package `arm-none-eabi-gcc`, for Ubuntu via instructions
+[here](https://launchpad.net/~team-gcc-arm-embedded/+archive/ubuntu/ppa), or
+see [here](https://launchpad.net/gcc-arm-embedded) for the main GCC ARM
+Embedded page.  The compiler can be changed using the `CROSS_COMPILE` variable
+when invoking `make`.
+
+To build for a given board, run:
+
+    $ make BOARD=PYBV11
+
+The default board is PYBV10 but any of the names of the subdirectories in the
+`boards/` directory can be passed as the argument to `BOARD=`.  The above command
+should produce binary images in the `build-PYBV11/` subdirectory (or the
+equivalent directory for the board specified).
+
+You must then get your board/microcontroller into DFU mode.  On the pyboard
+connect the 3V3 pin to the P1/DFU pin with a wire (they are next to each
+other on the bottom left of the board, second row from the bottom) and then
+reset (by pressing the RST button) or power on the board.  Then flash the
+firmware using the command:
+
+    $ make BOARD=PYBV11 deploy
+
+This will use the included `tools/pydfu.py` script.  You can use instead the
+`dfu-util` program (available [here](http://dfu-util.sourceforge.net/)) by
+passing `USE_PYDFU=0`:
+
+    $ make BOARD=PYBV11 USE_PYDFU=0 deploy
+
+If flashing the firmware does not work it may be because you don't have the
+correct permissions.  Try then:
+
+    $ sudo make BOARD=PYBV11 deploy
+
+Or using `dfu-util` directly:
+
+    $ sudo dfu-util -a 0 -d 0483:df11 -D build-PYBV11/firmware.dfu
+
+Accessing the board
+-------------------
+
+Once built and deployed, access the MicroPython REPL (the Python prompt) via USB
+serial or UART, depending on the board.  For the pyboard you can try:
+
+    $ picocom /dev/ttyACM0