Created SDRangel server (markdown)

SDRangel-server.md

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+As of now (since v3.9.0) this is an experimental proof of concept. It should be fully functonal but with limited support:
+
+Sampling devices:
+* HakRF (Rx/Tx)
+* LimeSDR (Rx/Tx)
+* RTLSDR
+
+Channel demodulators:
+* NFM
+
+Channel modulators:
+* NFM
+
+It is implemented as a new binary `sdrangelsrv` that is similar to `sdrangel` but does not fire up a GUI. This server mode binary has therefore no dependency on Qt Widgets nor OpenGL. Since ot has no GUI its control is left entirely up to the web REST API interface.
+
+It can serve two purposes:
+* running headless on systems with lower CPU power than a laptop or desktop computer like the Raspberry Pi3. Thanks to the web API it can be remotely controlled also and thus can be at the heart of a complex multi mode transponder or repeater infrastructure.
+* the web REST API can be used in a client-server configuration with the client implementing a custom and possibly simplified GUI. In this configuration the standard GUI becomes useless and can be removed to save CPU.
+
+This is distributed only in Linux packages.