Update Readme.md

I can count on one hand bioinformatics programs I actually like, and with these new arbitrary kmer sizes, swig, and bloom filters, Jellyfish is about to make it there. Really nice work.

In general Ruby users expect to install by using Rubygems, not by compiling from source. I'd be happy to make that happen, if you like. It would also be good to get it listed on http://biogems.info