RTL-SDR bias tee. Updated documentation

plugins/samplesource/rtlsdr/readme.md

@@ -52,6 +52,10 @@ These buttons control the local DSP auto correction options:
   - **DC**: auto remove DC component
   - **IQ**: auto make I/Q balance. The DC correction must be enabled for this to be effective.
 
+<h3>3a: Antenna bias tee</h3>
+
+Activates or de-activates the antenna bias tee. Works with v3 dongles only it will be simply ignored by others. It actually activates or de-activates GPIO pin 0 that commands bias tee on v3 dongles.
+
 <h3>4: Decimated bandpass center frequency position relative the RTL-SDR center frequency</h3>
 
   - **Cen**: the decimation operation takes place around the RTL-SDR center frequency Fs