added CDN & Right-to-Left CSS info

+ links to CDNs for jsDelivr & cdnjs
+ explained introjs-rtl.min.css

I also assumed the minfied files; let me know if you want to explicitly explain the readable code is without .min, but I think 99.999% of your users know this already ;)

BTW, I hand-made the CDN pull requests; still figuring out how to automate.
jsDelivr is a newer CDN, but has [more POPs](http://www.jsdelivr.com/network.php)