Add readme note about MONGO_INITDB_ROOT

[thespad]

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• [x] I have read the contributing guideline and understand that I have made the correct modifications

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Description:

Benefits of this PR and context:

How Has This Been Tested?

Source / References:

[LinuxServer-CI]

I am a bot, here is the pushed image/manifest for this PR:

ghcr.io/linuxserver/lspipepr-unifi-network-application:7.5.174-pkg-d75cd9cc-dev-bbad9795515b79f8ffca89674bafe99064bbc710-pr-3