Add comparison to css-modules in BENEFITS.md

There should be a note [here](https://github.com/callstack/linaria/blob/master/docs/BENEFITS.md) about css-modules. Because, as far as I can tell, there's no advantage to using this tool over css-modules. If you can't write dynamic styles, then what's the point of writing your CSS code in JavaScript in the first place? Why not just use CSS-Modules? The tooling already exists for it (`css-loader`), it works better with CSS linters and code highlighters, and it's easier to read by folks familiar with CSS.

I could be mistaken though, hence the need for a note about it. ;)