Update documentation

Add information on how to make use of custom
firmware.

Author: chrisruk

docs/buildhat/index.rst

@@ -28,10 +28,51 @@ power supply. For best results, use the `official Raspberry Pi Build HAT power s
 
 .. _official Raspberry Pi Build HAT power supply: http://raspberrypi.com/products/build-hat-power-supply
 
+It is now possible to use custom firmware with the library.  To do this you can follow the steps below.
+
+.. code-block::
+   :caption: Using custom firmware
+
+      sudo apt install cmake python3 build-essential gcc-arm-none-eabi libnewlib-arm-none-eabi libstdc++-arm-none-eabi-newlib
+
+      git clone https://github.com/raspberrypi/pico-sdk.git --recursive
+      git clone https://github.com/raspberrypi/buildhat.git --recursive
+
+      cd buildhat
+      export PICO_SDK_PATH="$(pwd)/../pico-sdk/"
+      make
+
+      cd ..
+      mkdir test
+      cd test
+      mkdir data
+
+      cp ../buildhat/firmware-pico/build/main.bin data/firmware.bin
+      cp ../buildhat/bhbl-pico/signature.bin data/signature.bin
+      cat ../buildhat/firmware-pico/version.h | sed 's/#define FWVERSION "//g; s/ .*//g' > data/version
+
+Then place your script, such as the following, within the test/ directory.
+
+.. code-block::
+   :caption: Create test.py in test directory
+
+      import time
+      from buildhat import Motor
+
+      m = Motor('A')
+      m.start()
+
+      time.sleep(5)
+
+Then use: ``python test.py`` in the test directory, to run your script with your custom firmware.
+
+Note if you want python to always reload the firmware from your **data/** directory each time
+you run your script, simply write the value: -1 to **data/version**.
+
 .. warning::
 
    The API for the Build HAT is undergoing active development and is subject
-   to change. 
+   to change.
 
 .. toctree::
    :maxdepth: 2