Add link to Docker image in README

[chrisn]

See issues #124 and #133. @realies - does this look OK to you?

[realies]

@chrisn, looks great, thanks for adding it!

[Grumpier]

Hi Realies. Thanks for creating this Docker image. I was hoping to use it from inside a nodejs app where I could stream audio from another source into audiowaveform. I've done a simple command line test and it seems your docker version closes the container without accepting the stream. Here's the output:

% cat 18_La_Bamba.mp3 | docker run -v "$(pwd):/tmp" -w /tmp realies/audiowaveform --input-format mp3 -o output.png
Input file: (stdin)
Done: 00:00
Frames decoded: 0 (0:00.000)
Invalid sample rate: 0 Hz

And with audiowaveform...

% cat 18_La_Bamba.mp3 | audiowaveform --input-format mp3 -o output.png
Input file: (stdin)
Format: Audio MPEG layer III stream
Bit rate: 128000 kbit/s
CRC: no
Mode: joint (MS/intensity) stereo
Emphasis: no
Sample rate: 44100 Hz
Encoding delay: unknown
Padding: unknown
Generating waveform data...
Samples per pixel: 256
Input channels: 2
Output channels: 1
Done: 02:52
Frames decoded: 6615 (2:52.800)
Generated 29768 points
Image dimensions: 800x250 pixels
Channels: 1
Sample rate: 44100 Hz
Samples per pixel: 256
Start time: 0 seconds
Start index: 0
Buffer size: 29768
Axis labels: yes
Amplitude scale: 1
Output file: output.png
Done

Any way you can fix this? Thanks!