docs(css): added info on grids

guides/css-styleguide.md

@@ -98,6 +98,15 @@ in useful diffs and blames.
 
 In order to create consistent class names, it's encouraged to use the naming defined at [semantic-ui.com](http://semantic-ui.com). This is especially true for components that are meant to be re-usable like icons or alert boxes.
 
-## Common Classes
+## Frameworks
 
-Use grids/common CSS class names to limit duplication in CSS...
+### Grids/Layout
+
+Encore UI currently does not prescribe a specific grid system that apps should use. We leave the choice of framework up to the developers of the app and what fits them best.
+
+For layout needs (e.g. rxApp and rxForm), Encore uses either `flexbox` and `floats`, respectively. While relatively new, flexbox has proven to be very powerful and will likely replace floats as the defacto solution for layouts in the Encore UI framework.
+
+### Recommended Libraries
+
+- http://purecss.io/grids/
+- http://flexboxgrid.com/