Add vignette how to do feature selection

[frederikziebell]

Since the original publication is called Feature selection and dimension reduction for single-cell RNA-Seq based on a multinomial model, it would be nice to have a part of the vignette explaining how to do feature selection with glmpca.

[willtownes]

Thanks for the suggestion, we ended up implementing the feature selection and residuals approximations in a separate package: https://www.bioconductor.org/packages/release/bioc/html/scry.html

I will leave this open as a reminder to myself to link to scry in the glmpca vignette.