Document that not all .pylintrc features are supported

[themanifold]

Hello,

I spent some time trying to figure out why ignored-classes in my .pylintrc file was being ignored when running pytest-pylint, and after trawling through the source code, I discovered that only a certain subset of features is supported by pytest-pylint.

What I'm trying to say is that we should probably make it really clear that we support only a few features in .pylintrc to save people grief.

This is only confusing because we let people specify a config file with no provisos, implying that we support all the features (but we don't).

Thanks.