Add missing placeholder in forms guide (vuejs#339)

miedzinski · kazupon · commit 7964824156bb · 2016-07-21T09:31:05.000+09:00
diff --git a/src/guide/forms.md b/src/guide/forms.md
@@ -37,7 +37,7 @@ new Vue({
 <span>Multiline message is:</span>
 <p>{{ message }}</p>
 <br>
-<textarea v-model="message"></textarea>
+<textarea v-model="message" placeholder="add multiple lines"></textarea>
 ```
 
 {% raw %}
