Add an Example of Setting Up GregTech: New Horizons

[elizabethlfransen]

GregTech: New Horizons requires a little bit of extra setup because it's a generic pack.

This PR adds an example of how to configure it

[elizabethlfransen]

I would have liked to use java options and CUSTOM_SERVER but unfortunately CUSTOM_SERVER requires the jar file to exist before the generic pack is downloaded

[elizabethlfransen]

I'm not sure if putting @java9args.txt in CUSTOM_JAR_EXEC will work, but I was able to run the server and connect to it.

[itzg]

Ah yes, that is quite particular. Thanks for providing that!

[itzg]

I would have liked to use java options and CUSTOM_SERVER but unfortunately CUSTOM_SERVER requires the jar file to exist before the generic pack is downloaded

Yeah I'll have to remember why I didn't relax that in the first place. It seems like it would be reasonable to relax it at least when GENERIC_PACK is used.