add line to install pcaudiolib on debian

Literally following the instructions on a base install of Debian results in a build without audio support. This change adds the instruction for installing pcaudiolib development headers, so that the resulting build has audio.

README.md

@@ -125,6 +125,7 @@ be installed using the following commands:
 | sonic         | `sudo apt-get install libsonic-dev`                              |
 | ronn          | `sudo apt-get install ruby-ronn`                                 |
 | kramdown      | `sudo apt-get install ruby-kramdown`                             |
+| pcaudiolib    | `sudo apt-get install libpcaudio-dev`                            |
 
 ### Building