Slight tweak to README.md

Thanks a million for providing this resource, by the way!!! I tried to pull all the fonts with Mercurial, but it kept failing (slightly unstable Internet connection)... this is so much more convenient!

I noticed that the README said this repo is ~60MB, but `du` tells me that my freshly cloned repo takes 244MB on disk. Perhaps the number of fonts has grown a lot over time? Anyways, I just tweaked the README slightly to make it more accurate.