Update README.md

I just got some cryptic errors trying to get Jekyll working again on an upgraded OS X 10.10 system. The Jekyll install instructions didn't really help--they worked fine, but when it came time to serve up the CUGOS pages, lots of errors. After lots of fiddling, I realized I needed to manually install jekyll-sitemap. You really don't need to merge this request if it breaks up the flow or is too idiosyncratic, but I thought in the spirit of offering help to others who don't know much about Jekyll, I would at least offer the change...