Add README and testfile

README.md

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+# Application note on using rpitx
+
+## Using with csdr to modulate streaming input
+
+You will need the *dev* branch of *csdr* for doing this.<br />
+Setup instructions:
+
+   	git clone https://github.com/simonyiszk/csdr.git
+   	cd csdr
+   	git fetch
+   	git checkout dev
+   	make && sudo make install 
+
+Note that it should be already done, if you installed *qtcsdr*.
+
+### Modulate from raw audio file
+
+Generate AM modulation:
+
+    cat testsound48000.raw | csdr convert_i16_f | csdr gain_ff 1.0 | csdr dsb_fc | csdr add_dcoffset_cc | sudo rpitx -i- -m IQFLOAT -f 28400
+
+* It will read the raw audio file `testsound48000.raw`, which is present in this repo. You can get this file by:
+
+    git clone https://github.com/ha7ilm/rpitx-app-note
+	cd rpitx-app-note; ls  #There is your file.
+
+* The part `csdr gain_ff 1.0` can be changed to increase/decrease modulation.
+* The `-f 28400` at the end gives the transmit frequency in kHz.
+
+
+
+
+