add returning promise example for returns tag

Author: zhengzongyi

content/en/tags-returns.md

@@ -73,3 +73,20 @@ function sum(a, b, retArr) {
 }
 ```
 {% endexample %}
+
+{% example "Returns a promise" %}
+
+```js
+/**
+ * Returns the sum of a and b
+ * @param {Number} a
+ * @param {Number} b
+ * @returns {Promise<Number>} Promise object represents the sum of a and b
+ */
+function sumAsync(a, b) {
+    return new Promise(function(resolve, reject){
+        resolve(a + b);
+    });
+}
+```
+{% endexample %}

tags-returns.html

@@ -90,8 +90,20 @@ <h2 id="examples">Examples</h2>
     }
     return a + b;
 }
-</code></pre>
-    </figure>
+</code></pre></figure>
+    <figure>
+      <figcaption>Returns a promise</figcaption><pre class="prettyprint lang-js"><code>/**
+ * Returns the sum of a and b
+ * @param {Number} a
+ * @param {Number} b
+ * @returns {Promise<Number>} Promise object represents the sum of a and b
+ */
+function sumAsync(a, b) {
+    return new Promise(function(resolve, reject){
+        resolve(a + b);
+    });
+}
+</code></pre></figure>
     <h2 id="related-links">Related Links</h2>
     <ul>
       <li>