add Vue.version into API (vuejs#755)

* add Vue.version into API

* Update Vue version API

Author: vinicius73

src/v2/api/index.md

@@ -329,6 +329,24 @@ type: api
   ```
 
 - **See also:** [Render Functions](../guide/render-function.html)
+ 
+<h3 id="Vue-version">Vue.version</h3>
+
+- **Details**: Provides the installed version of Vue as a string. This is especially useful for community plugins and components, where you might use different strategies for different versions.
+
+- **Usage**:
+
+```js
+var version = Number(Vue.version.split('.')[0])
+
+if (version === 2) {
+  // Vue v2.x.x
+} else if (version === 1) {
+  // Vue v1.x.x
+} else {
+  // Unsupported versions of Vue
+}
+```
 
 ## Options / Data