Add some useful stuff for the video team

useful-stuff.md

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+# Useful Stuff
+
+This file contains information, links, tools/commands that can be useful for the video team.
+
+## Testing
+
+### Test Presentation
+
+http://tochev.net/presentation-test/
+
+### Sample RTMP Stream
+
+```sh
+ffmpeg -re -f lavfi -i testsrc=size=1920x1080:rate=30 -f lavfi -i "sine=frequency=220" -c:v libx264 -tune zerolatency -af "volume=1.0" -c:a aac -f flv rtmp://localhost/test.stream
+```
+
+The above command will send a 1080p test pattern with sine audio.
+
+## Tools
+
+### Play Video with Audio Bar
+
+```sh
+mpv --mute --demuxer-lavf-analyzeduration=30 --lavfi-complex='[aid1] asplit [t1] [ao] ; [t1] showvolume=w=1000:h=100 [t2] ; [vid1]  [t2]  overlay  [vo]' SOME_VIDEO_SOURCE
+```
+
+Remove `--mute` if you want to start the video with an audible audio.
+
+### ffprobe
+
+```sh
+ffprobe SOME_VIDEO_SOURCE
+```
+
+Test the resolution, length, codec, etc. of a video source.
+
+## Resources
+
+### Royalty-Free Music
+
+- https://pixabay.com/music/ (do check the license of the file)