respond.js does not work in IE8 w/ CDN

[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.

[cvrebert]

X-Ref: #10989

[ysimonson]

Probably not worth mentioning in the doc, but did come across another issue that sapped some time: scottjehl/Respond#229

[cvrebert]

We also should include a note whenever we refer to our local copy in our examples.

[farneman]

Maybe the docs just need to include a general note to refer to the Respond.js docs for help with media query troubles in IE8 and below? Otherwise the Bootstrap docs would probably end up replicating most of the Respond docs over time which doesn't seem maintainable or efficient.

[farneman]

A general note about would also help resolve #11019 and some similar pull requests as well.