Add ffmpeg mod info to README

[mKeRix]

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We welcome all PR’s though this doesn’t guarantee it will be accepted.

Description:

Added info about the ffmpeg mod to the README as requested.

Benefits of this PR and context:

LazyLibrarian has added support to do conversion and tagging of audiobooks which requires ffmpeg to be available on the system. This mod installs it into the container on startup, which makes the conversion features functional. Closes #45.

How Has This Been Tested?

Enabled on personal LazyLibrarian instance, running on Docker 19.03.8/Ubuntu. ffmpeg is installed and recognized by LazyLibrarian.

Source / References:

Original PR for the new mod: linuxserver/docker-mods#38

[CHBMB]

@mKeRix Sorry to keep asking you to do more work, but all our README.md files are generated from the readme-vars.yml file, so unfortunately the changes you've made would be overwritten once the image is updated by Jenkins.

If you're happy to make the changes to readme-vars.yml then that would be awesome, if not let me know and I can submit a PR.

https://github.com/linuxserver/docker-lazylibrarian/blob/master/readme-vars.yml

[mKeRix]

Oh sorry, didn't see that. Will revert the README and change the vars now. Thanks for the heads up!

[mKeRix]

I migrated the content from the README to readme-vars, if I can improve the texts somehow feel free to let me know. I'm not sure what caused the PR build to fail though - it doesn't look like the step that failed used the readme-vars in anyway.

[mKeRix]

@CHBMB Anything I can do to push this PR along further? :)

[LinuxServer-CI]

I am a bot, here are the test results for this PR:
https://lsio-ci.ams3.digitaloceanspaces.com/lspipepr/lazylibrarian/7cf8ec40-pkg-9a77cbf9-pr-47/index.html
https://lsio-ci.ams3.digitaloceanspaces.com/lspipepr/lazylibrarian/7cf8ec40-pkg-9a77cbf9-pr-47/shellcheck-result.xml