arkflag documentation

[YodaDaCoda]

After a bunch of testing and reading through the code of arkmanager, it looks like the value of an am_arkflag_* environment variable doesn't matter, only its existance (perhaps it needs to be set to something rather than simply existing, but the value doesn't matter as far as I can tell).
E.g. setting am_arkflag_NoBattleEye=false in my compose file results in the -NoBattleEye flag being set when starting the server.

IMO arkmanager should handle this more intelligently, but in the meantime the readme should be updated to reflect.

[jkread]

Interesting I'll have to test this out.