Add note for those of us asking "Where has pmx.sample gone?"

Hi Dan. Hope you're well!

I was a big user of `pymc3_ext.sample`. One for the `parameter_groups` keyword which allowed grouping of covariant parameters, and two for the `regularization_steps` parameter which helped a lot when sampling big datasets with correlations.

Would it be possible for you to explain, here or in the ReadMe, what happened to those features? Are they now part of the vanilla `pymc` functionality? Or are you still working on getting them ported over from `pymc3_ext` to here?

Thanks in advance!