Document "spotless.apply.skip" and "spotless.skip" in README

[kwin]

Currently the README only explains the parameter spotless.check.skip but not the equivalent for format, namely spotless.format.skip. Currently the README may be read as if spotless.check.skip would apply to all goals. The goal agnostic skip, namely spotless.skip is not described at all.

Also for documentation reason it would be good to only make parameters spotlesss.check.skip and spotless.apply.skip belong to the respective plugins only instead of the shared super class.