Mention FabricMC repo in README

[Tom-The-Geek]

It might be a good idea for the README to mention that you may have to add the Fabric maven as well as the repo for the extension. Maybe something like

_Note: You may also have to add the FabricMC maven repo as well if dependencies fail to resolve (url: `https://maven.fabricmc.net`)_

underneath the maven repo URL and artifact coordinates?

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[gdude2002]

Thanks for the heads up!