Add description about ZenHub Browser Extension

Author: y-yagi

CONTRIBUTING.md

@@ -74,6 +74,7 @@ again.
 ### Issue management
 
 * We use [Zenhub](https://www.zenhub.com) to manage the queue, in addition to what we do with labels.
+  * You will need to install [ZenHub extension](https://www.zenhub.com/extension) to your browser to show the board.
   * Pipelines, and [the board](https://github.com/golang/dep#boards) are one thing we try to utilize:
     * **New Issues Pipeline**: When someone creates a new issue, it goes here first. Keep an eye out for issues that fall into your area. Add labels to them, and if it's something we should do, put it in the `Backlog` pipeline. If you aren't sure, throw it in the `Icebox`. It helps to sort this pipeline by date.
     * **Icebox Pipeline**: Issues that we aren't immediately closing but aren't really ready to be prioritized and started on. It's not a wontfix bucket, but a "not sure if we should/can fix right now" bucket.