Add opus sample frame generation GStreamer command to the readme

[sirknightj]

Issue #, if available:

• N/A

What was changed?

• The readme has been updated with a sample GStreamer pipeline to generate some sample Opus frames.

Why was it changed?

• The information did not exist in the readme and it is useful information for users to know to experiment.

How was it changed?

• Added a sample command using audiotestsrc GStreamer element.
• The new section was placed next to similar spot for the video frames generation.
• Note that I was not able to locate the original music that it was generated from. It is likely the original author used filesrc and a local file instead of audiotestsrc.

What testing was done for the changes?

• Locally replaced the sample frames and checked the playback with the JS page.

I also was able to get an mp3 audio to be played back on the JS page as well:

gst-launch-1.0 filesrc location="./My-Cool-Audio.mp3" ! decodebin ! audioconvert ! audioresample ! opusenc ! audio/x-opus,rate=48000,channels=2 ! multifilesink location="sample-%03d.opus" index=1

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[stefankiesz]

I confirmed these sample audio files stream e2e using Master sample and JS Viewer.