Add warning about using scoped in place of classes (#206)

Author: chrisvfritz

docs/features/scoped-css.md

@@ -46,4 +46,6 @@ Into the following:
 
 3. Partials are not affected by scoped styles.
 
-4. **Be careful with descendant selectors in recursive components!** For a CSS rule with the selector `.a .b`, if the element that matches `.a` contains a recursive child component, then all `.b` in that child component will be matched by the rule.
+4. **Scoped styles do not eliminate the need for classes**. Due to the way browsers render various CSS selectors, `p { color: red }` will be many times slower when scoped (i.e. when combined with an attribute selector). If you use classes or ids instead, such as in `.example { color: red }`, then you virtually eliminate that performance hit. [Here's a playground](http://stevesouders.com/efws/css-selectors/csscreate.php) where you can test the differences yourself.
+
+5. **Be careful with descendant selectors in recursive components!** For a CSS rule with the selector `.a .b`, if the element that matches `.a` contains a recursive child component, then all `.b` in that child component will be matched by the rule.