Add info about MiniCssExtractPlugin

[denisinvader]

I've spent half of my workday trying to render my app on a server and didn't understand what's wrong. The problem was in MiniCssExtractPlugin because it doesn't care about SSR.

I had document is not defined error at first. I've tried to mock document in NodeJs using JSDOM (I understood that something wrong if I need to do it) and the error has gone, but nothing just rendered, router.onReady didn't resolve anything.

Many hours of searching later I've accidentally found some issue (can't provide a reference) where someone said that MiniCssExtractPlugin doesn't care about SSR. Then I've switched to ExtartTextPlugin and everything worked.

So please add a note about this plugin in docs.

[LinusBorg]

Your post assumes we are ware of the problem you decribe with:

The problem was in MiniCssExtractPlugin because it doesn't care about SSR.
[...]
Many hours of searching later I've accidentally found some issue (can't provide a reference)

I'm not aware of the issue, and now I neither know what the actual problem is, nor how to reproduce it, and consequently, don't know what eactly to document.

I won't write in the docs: "MiniCSSExtractPlugin doesn't care about SSR" and nothing else.

[LinusBorg]

Closing in favor of #187