respond.js does not work in IE8 w/ CDN

Created by: ysimonson

Hi, I lost a lot of time trying to figure out why our CDN setup with bootstrap 3 didn't work in IE8. It turns out respond.js requires a special setup when using a CDN, as explained in its readme: https://github.com/scottjehl/Respond/blob/master/README.md

I think these instructions should be included in the "supported browsers" section of bootstrap's documentation so others don't have to go through the nine circles of debugging hell like I had to.