Add README for Quisk Software Defined Radio

Added introductory information about Quisk software defined radio, including its features and support links.

README.md

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+# Quisk Software Defined Radio
+
+Quisk is a software defined radio transceiver that runs on a PC or on a single board computer such as a Raspberry Pi.
+It is written in Python and C.
+It works with various kinds of radio hardware such as SoftRock, Hermes Lite 2,
+the SDR-IQ by RfSpace, and with network UDP samples.
+Quisk is easily extensible by writing your own hardware file in Python.
+
+I wrote Quisk back in 2008, and I have been maintaining it ever since.
+For questions and support, please visit the group https://groups.io/g/n2adr-sdr.
+
+For more information see https://james.ahlstrom.name/quisk.