Expand README.md to a short manual

README.md

 # R128
-This application measures loudness according to EBU R128 with ffmpeg.
+This application measures loudness according to EBU R128 with ffmpeg and ffprobe.
 
 You can send video or audio files to the app.
 Channel layout inside video can be mono or stereo. But mono files will temporarily merge to a stereo file at first.
 If number of mono tracks is uneven it will return an error.
-Temporary output.wav is exported to %temp% in 24bit 48kHz.
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+
+Send a file to R128. 
+(If opening with double click you get a file select window.)
+If there are more then one stereo or two mono tracks you get a popup to choose the tracks.
+
+![select tracks](/images/select_tracks.png)
+
+The choosen tracks are first extracted to `%temp%` as `output.wav` in 24bit and 48kHz.
+Progress is shown in the gui.
+Then it measures this wave file. If ready you can copy the result to the clipboard.
+
+![GUI](/images/gui.png)
+
+`Output.wav` will stay at `%temp%` until overridden.
+So you can use the app as an audio extractor or audio converter too.