Add info on XDG_CONFIG_HOME

[ysthakur]

Adds information on using XDG_CONFIG_HOME to control where Nushell looks for config files. Also added a warning about setting XDG_CONFIG_HOME to the directory above the Nushell-specific config directory, rather than the nushell directory itself.

[fdncred]

We might want to have another warning that says, "Do not set your XDG_CONFIG_HOME in env.nu". I know you emphasized before but I think a warning and a more verbose statement of "do not do this" should be there too.

[fdncred]

Thanks!