Support for $HOME/.config/asdf/asdfrc?

The nature of this request depends on whether you already support keeping asdf's rc file inside `$HOME/.config` or not.

If so, that's awesome. Would you consider adding that to the README (mentioning that location along with `$HOME/.asdfrc`)?

If not, then would you consider adding support for that?

Or alternately, if you don't have time to add support for that, would you accept a PR that added such support?