Minor update to documentation

[erinMahoney]

Hello love this package. I've used it for work, and it's been very successful at addressing our use-case. I have a small request/recommendation. The doc string here indicates that redundancy should return a pd.Series object--we found that another requirement is that the object should have an index with the feature values. Without it, this chunk will update the redundancy dataframe to np.nan values.

Happy to make the adjustment and PR myself if you agree.