Add documentation for importLoaders

[AndrewRayCode]

Currently this is in the README for importLoaders documentation:

The query parameter importLoaders allow to configure which loaders should be applied to @imported resources.

importLoaders (int): That many loaders after the css-loader are used to import resources.

This is confusing and hard to understand. After reading this, I have no idea what importLoaders does. In fact I think I walk away more confused than before reading the docs.

I would happily submit an improvement to make this comprehensible if I could figure out what this option does.

[zhouzi]

There's an other issue opened with some interesting insights here: #228

[bebraw]

Let's close in favor of #228.