Auto merge of #27386 - asajeffrey:readme-gstreamer-packages, r=Manishearth
Auto merge of #27386 - asajeffrey:readme-gstreamer-packages, r=Manishearth

Added debian gstreamer packages to README

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Document which gstreamer packages are needed on Ubuntu 20.04LTS.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #27363
- [x] These changes do not require tests because it's edits to the README

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