add some help to Readme

README.md

-# Common FeM NixOS Config
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+# Common FeM NixOS Config
+To use this repo in your Config simply execute this in your Repo:
+
+`git submodule add -b main git@gitlab.fem-net.de:nixos/fem-common.git fem`
+
+This will clone this repo to `./fem`
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