doc: document variants

Author: andreypopp

README.md

@@ -47,9 +47,25 @@ And consume them like regular React components:
 
     <Label /> // => <div className="<autogenerated classname>">...</div>
 
+## Variants
+
+You can define additional styling variants for your components:
+
+    Label {
+      color: red;
+    }
+
+    Label:emphasis {
+      font-weight: bold;
+    }
+
+They are compiled as CSS classes which then can be controlled from JS via
+`variant` prop:
+
+    <Label variant={{emphasis: true}} /> // sets both classes with `color` and `font-weight`
+
 ## TODO
 
-* [ ] Support variants (generic pseudoclasses which can be toggled via JS).
 * [ ] Support adding PostCSS transform to build pipeline (think autoprefixer).
 
 [CSS modules]: https://github.com/css-modules/css-modules