docs: add recommended packages

README.md

@@ -29,6 +29,19 @@ There is no longer any dependency on SphinxBase.  There is no
 SphinxBase anymore.  This is not the SphinxBase you're looking for.
 All your SphinxBase are belong to us.
 
+There are some other dependencies that you may find useful in order to
+use the example code (though they are not strictly necessary to build
+and install).  On Debian GNU/Linux and its derivatives (such as
+Raspberry Pi OS, Ubuntu, etc), you can install them with:
+
+    sudo apt install \
+        ffmpeg \
+        libasound2-dev \
+        libportaudio2 \
+        libportaudiocpp0 \
+        libpulse-dev \
+        portaudio19-dev
+
 To install the Python module in a virtual environment (replace
 `~/ve_pocketsphinx` with the virtual environment you wish to create),
 from the top level directory: