Create readme.md

Author: swharden

src/Spectrogram/readme.md

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+**Spectrogram** is a .NET library which makes it easy to create spectrograms from pre-recorded signals or live audio from the sound card.
+
+### Quickstart
+
+```cs
+// load audio and process FFT
+var spec = new Spectrogram.Spectrogram(sampleRate: 8000, fftSize: 2048, step: 700);
+float[] values = Spectrogram.Tools.ReadWav("mozart.wav");
+spec.AddExtend(values);
+
+// convert FFT to an image and save it
+Bitmap bmp = spec.GetBitmap(intensity: 2, freqHigh: 2500);
+spec.SaveBitmap(bmp, "mozart.jpg");
+```
+
+![](https://raw.githubusercontent.com/swharden/Spectrogram/master/data/mozart.jpg)
+
+### Additional Resources
+Much more is on the Spectrogram project page:\
+**[https://github.com/swharden/Spectrogram](https://github.com/swharden/Spectrogram)**