Add guide link in Vue.filter API (#1394)

Author: william-pan

src/v2/api/index.md

@@ -323,6 +323,8 @@ type: api
   var myFilter = Vue.filter('my-filter')
   ```
 
+- **See also:** [Filters](../guide/filters.html)
+
 ### Vue.component( id, [definition] )
 
 - **参数**：