add audio/shairport-sync

Shairport Sync is an AirPlay audio player - it plays audio streamed from
Apple devices and AirPlay sources such as ForkedDaapd (but apparently not
rtunes).

Audio played by a Shairport Sync-powered device stays synchronised with the
source and hence with similar devices playing the same source. In this way,
synchronised multi-room audio is possible for players that support it.

(nia)