Add readme note about iptables modules

[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:

See #266

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-wireguard:v1.0.20210914-pkg-f322ed89-dev-cf0aeaf175ead6347bb5d128f2176990d731e41a-pr-267