Update 'README.md'

README.md

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 This handy repository will make it easy to get a script or config file on a server
 
 Please take care NOT to push scripts with sensitive information in them.
+
+### How to Use prep-grid-vm.sh
+- To install only Docker, you would run: ./prep-grid-vm.sh -d
+- To install only Caddy, you would run: ./prep-grid-vm.sh -c
+- To install both Docker and Caddy, you would run: ./prep-grid-vm.sh -dc