No TimeGrouper docs, Day-of-Week resampling?
TimeGrouper isn't really mentioned in the docs at all. In particular, it'd be nice to know what the grouping options are. I assume they're the same as resample's options? I also can't find a simple list of those.

In particular, weekly grouping is doable via `'W'`, but I can't find that listed in the docs anywhere.

It would also be really nice to have a `'DoW'` (Day of Week) grouping option - this can be really useful for any data with a human signal in it.