Clarify how to customise output directory

[goulvench]

Hi,

Thanks for this gem that helped me get a better view of the domain model.

I was able to generate the file in a directory by passing filename="docs/ERD" to the command-line, but this wasn't very clear, neither from the option name nor from the documentation.
I think that it would be useful to accept a path option, or document that the filename option can include a path.