Add engines guide to documents.yaml

railties/guides/source/documents.yaml

@@ -96,6 +96,10 @@
       name: Asset Pipeline
       url: asset_pipeline.html
       description: This guide documents the asset pipeline.
+    - 
+      name: Getting Started with Engines
+      url: engines.html
+      description: This guide explains how to write a mountable engine.
     - 
       name: The Rails Initialization Process
       work_in_progress: true