Link to RuboCop docs is out of date

[dphdave]

At https://rubocop-sketchup.readthedocs.io/en/latest/configuration/, in "Refer to the RuboCop documentation about general configuration of RuboCop itself."
the provided link appears to be out of date, please replace with eg

https://github.com/rubocop/rubocop/blob/master/README.md
or
https://rubocop.org/

[thomthom]

PR welcome ;)