Pivot facets not excluded

[cdmo]

I had to manually unset pivot facets from the blacklight advanced request parameters. I did this by setting pivot_facet to ''. I'm a) not sure that it is expected behavior that pivot facets can't be excluded the same way as other facets and b) not sure the way I excluded it is the best way (though it worked).

I believe this is a partially known issue too.

If this is expected behavior, I think that it'd be useful to add a note to the README on pivot facets.