Add a command for converting the WAV files

Add a command for converting the WAV files to the amp skin tutorial, so that people can use their own files easily.

docs/tutorial/amp_skin.rst

@@ -55,9 +55,13 @@ For example::
 You can also play WAV files using the Python ``wave`` module.  You can get
 the wave module `here <http://micropython.org/resources/examples/wave.py>`_ and you will also need
 the chunk module available `here <http://micropython.org/resources/examples/chunk.py>`_.  Put these
-on your pyboard (either on the flash or the SD card in the top-level
-directory).  You will need an 8-bit WAV file to play, such as
-`this one <http://micropython.org/resources/examples/test.wav>`_.  Then you can do::
+on your pyboard (either on the flash or the SD card in the top-level directory).  You will need an
+8-bit WAV file to play, such as `this one <http://micropython.org/resources/examples/test.wav>`_,
+or to convert any file you have with the command::
+
+    avconv -i original.wav -ar 22050 -codec pcm_u8 test.wav
+    
+Then you can do::
 
     >>> import wave
     >>> from pyb import DAC