Merge pull request #240 from hootlex/patch-1

add 'What are Components' section

Author: yyx990803

src/guide/components.md

@@ -4,6 +4,11 @@ type: guide
 order: 12
 ---
 
+##What are Components?
+Components are one of the most powerful features of Vue.js, helping you extend basic HTML elements and create reusable code. At a high level, Components are custom elements that Vue.js’ compiler would attach specified behavior to, or even transform the elements and their children.
+
+In some cases, a Component is indicated with a special attribute on an HTML element.
+
 ## Using Components
 
 ### Registration