Added documentation pointer for audio device sharing on Linux

doc/Audio.txt

+
+Audio device sharing on Linux
+-----------------------------
+
+Some audio devices support hardware mixing, these device can be opened
+multiple times from different application natively. If this is not the case,
+you will notice that is not possible to use SIP SIMPLE client SDK and play
+music or use other audio applications at the same time with the default
+system configuration.
+
+For audio devices that can be opened only once, there are workarounds. More
+information about them can be found at:
+
+http://sipsimpleclient.com/wiki/SipAudioDeviceSharingLinux
+