Small suggestion for sc3_prepare() documentation #80

Hi,

Thanks for your work developing SC3!

It might be worth noting that sc3_prepare() adds the sc3_gene_filter column to rowData(object) if gene_filter = TRUE (in addition to creating and populating the metadata(object)).
I had trouble tracking down an error (a user error on my part) because I ran sc3_estimate_k() before and after running sc_prepare() and obtained different values due to the gene_filter; I couldn't figure out how to reconcile the two result based on the docs but realised the source (and my error) once I compared the before/after objects using all.equal().

Cheers,
Pete