Add WIP Rails 5.1 release notes to documents.yaml

[ci skip]

guides/source/documents.yaml

@@ -193,6 +193,11 @@
       name: Upgrading Ruby on Rails
       url: upgrading_ruby_on_rails.html
       description: This guide helps in upgrading applications to latest Ruby on Rails versions.
+    -
+      name: Ruby on Rails 5.1 Release Notes
+      url: 5_1_release_notes.html
+      description: Release notes for Rails 5.1.
+      work_in_progress: true
     -
       name: Ruby on Rails 5.0 Release Notes
       url: 5_0_release_notes.html