Added ffmpeg's notes to the knowledge base

knowledge base/ffmpeg.md

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+# ffmpeg
+
+## TL;DR
+
+```shell
+# Convert a webm file to GIF.
+ffmpeg -y -i rec.webm -vf palettegen palette.png \
+  && ffmpeg -y -i rec.webm -i palette.png \
+       -filter_complex paletteuse -r 10 out.gif
+```
+
+## Format conversion
+
+### Webm to GIF
+
+```shell
+ffmpeg -y -i rec.webm -vf palettegen palette.png
+ffmpeg -y -i rec.webm -i palette.png -filter_complex paletteuse -r 10 out.gif
+```
+
+Here `rec.webm` is the recorded video.  
+The first command creates a palette out of the webm file. The second command converts the webm file to gif using the created palette.
+
+## Sources
+
+- [Convert a webm file to gif]
+- [How to convert a webm video to a gif on the command line]
+
+[convert a webm file to gif]: https://mundanecode.com/posts/convert-webm-to-gif
+[how to convert a webm video to a gif on the command line]: https://askubuntu.com/questions/506670/how-to-do-i-convert-an-webm-video-to-a-animated-gif-on-the-command-line