Add Mac-specific readme.

README-mac.txt

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+This is Gqrx 2.2 for Mac.
+
+Gqrx is a simple software defined radio receiver application for Linux and Mac, powered by GNU Radio and the Qt GUI toolkit.
+
+This binary distribution includes support for the following devices:
+- Funcube Dongle Pro and Pro+
+- RTL-SDR (via USB or TCP)
+- USRP devices from Ettus Research
+- HackRF Jawbreaker
+- OsmoSDR
+
+Plug in your device and start Gqrx. Your device should be detected and shown in the drop-down list. Note that some input devices (e.g. hackrf) will always be present in the drop-down list even if you don't have plugged in.
+
+The bundle also includes various command line utilities that come with the driver libraries. They are located in Gqrx.app/Contents/MacOS/
+
+Gqrx can also be started from the command line with various options that are useful for manipulating the configuration. Run 'Gqrx.app/Contents/MacOS/Gqrx -h' to get a list of options.
+
+Gqrx 2.2 for Mac should be considered beta software. Feedback is welcome: http://gqrx.dk/
+