Add reminder about using quotes with kebab-case (vuejs#676)

It may be obvious to many users but the guide seems to suggest that you can use unquoted kebab-case CSS property names. A reminder to use quotes here would be helpful since the "invalid expression" warning could be vague in many cases.

Author: regularmike

src/v2/guide/class-and-style.md

@@ -153,7 +153,7 @@ When `isActive` is truthy, the rendered HTML will be:
 
 ### Object Syntax
 
-The object syntax for `v-bind:style` is pretty straightforward - it looks almost like CSS, except it's a JavaScript object. You can use either camelCase or kebab-case for the CSS property names:
+The object syntax for `v-bind:style` is pretty straightforward - it looks almost like CSS, except it's a JavaScript object. You can use either camelCase or kebab-case (use quotes with kebab-case) for the CSS property names:
 
 ``` html
 <div v-bind:style="{ color: activeColor, fontSize: fontSize + 'px' }"></div>