Add note regarding explicit prop declaration requirement (vuejs#939)

Add note regarding explicit prop declaration requirement

Author: KKSzymanowski

src/v2/guide/components.md

@@ -696,6 +696,7 @@ Vue.component('my-checkbox', {
     event: 'change'
   },
   props: {
+    checked: Boolean,
     // this allows using the `value` prop for a different purpose
     value: String
   },
@@ -717,6 +718,8 @@ The above will be equivalent to:
 </my-checkbox>
 ```
 
+<p class="tip">Note that you still have to declare the `checked` prop explicitly.</p>
+
 ### Non Parent-Child Communication
 
 Sometimes two components may need to communicate with one-another but they are not parent/child to each other. In simple scenarios, you can use an empty Vue instance as a central event bus: