Add "Ruby on Rails 6.0 Release Notes" guide [ci skip]

This commit adds a skeleton of "Ruby on Rails 6.0 Release Notes".
It isn't a good time to add changelogs' entries to this guide since we can
redo/revert some things till the final release 6.0.
It would be better to do it close to the release.
But we already can add mentions about major features
that have been added to 6.0. I added mention about "Parallel Testing".

guides/source/6_0_release_notes.md

@@ -0,0 +1,175 @@
+**DO NOT READ THIS FILE ON GITHUB, GUIDES ARE PUBLISHED ON https://guides.rubyonrails.org.**
+
+Ruby on Rails 6.0 Release Notes
+===============================
+
+Highlights in Rails 6.0:
+
+* Parallel Testing
+
+These release notes cover only the major changes. To learn about various bug
+fixes and changes, please refer to the change logs or check out the [list of
+commits](https://github.com/rails/rails/commits/6-0-stable) in the main Rails
+repository on GitHub.
+
+--------------------------------------------------------------------------------
+
+Upgrading to Rails 6.0
+----------------------
+
+If you're upgrading an existing application, it's a great idea to have good test
+coverage before going in. You should also first upgrade to Rails 5.2 in case you
+haven't and make sure your application still runs as expected before attempting
+an update to Rails 6.0. A list of things to watch out for when upgrading is
+available in the
+[Upgrading Ruby on Rails](upgrading_ruby_on_rails.html#upgrading-from-rails-5-2-to-rails-6-0)
+guide.
+
+Major Features
+--------------
+
+### Parallel Testing
+
+[Pull Request](https://github.com/rails/rails/pull/31900)
+
+[Parallel Testing](testing.html#parallel-testing) allows you to parallelize your
+test suite. While forking processes is the default method, threading is
+supported as well. Running tests in parallel reduces the time it takes
+your entire test suite to run.
+
+Railties
+--------
+
+Please refer to the [Changelog][railties] for detailed changes.
+
+### Removals
+
+### Deprecations
+
+### Notable changes
+
+Action Cable
+------------
+
+Please refer to the [Changelog][action-cable] for detailed changes.
+
+### Removals
+
+### Deprecations
+
+### Notable changes
+
+Action Pack
+-----------
+
+Please refer to the [Changelog][action-pack] for detailed changes.
+
+### Removals
+
+### Deprecations
+
+### Notable changes
+
+Action View
+-----------
+
+Please refer to the [Changelog][action-view] for detailed changes.
+
+### Removals
+
+### Deprecations
+
+### Notable changes
+
+Action Mailer
+-------------
+
+Please refer to the [Changelog][action-mailer] for detailed changes.
+
+### Removals
+
+### Deprecations
+
+### Notable changes
+
+Active Record
+-------------
+
+Please refer to the [Changelog][active-record] for detailed changes.
+
+### Removals
+
+### Deprecations
+
+### Notable changes
+
+Active Storage
+--------------
+
+Please refer to the [Changelog][active-storage] for detailed changes.
+
+### Removals
+
+### Deprecations
+
+### Notable changes
+
+Active Model
+------------
+
+Please refer to the [Changelog][active-model] for detailed changes.
+
+### Removals
+
+### Deprecations
+
+### Notable changes
+
+Active Support
+--------------
+
+Please refer to the [Changelog][active-support] for detailed changes.
+
+### Removals
+
+### Deprecations
+
+### Notable changes
+
+Active Job
+----------
+
+Please refer to the [Changelog][active-job] for detailed changes.
+
+### Removals
+
+### Deprecations
+
+### Notable changes
+
+Ruby on Rails Guides
+--------------------
+
+Please refer to the [Changelog][guides] for detailed changes.
+
+### Notable changes
+
+Credits
+-------
+
+See the
+[full list of contributors to Rails](http://contributors.rubyonrails.org/)
+for the many people who spent many hours making Rails, the stable and robust
+framework it is. Kudos to all of them.
+
+[railties]:       https://github.com/rails/rails/blob/6-0-stable/railties/CHANGELOG.md
+[action-pack]:    https://github.com/rails/rails/blob/6-0-stable/actionpack/CHANGELOG.md
+[action-view]:    https://github.com/rails/rails/blob/6-0-stable/actionview/CHANGELOG.md
+[action-mailer]:  https://github.com/rails/rails/blob/6-0-stable/actionmailer/CHANGELOG.md
+[action-cable]:   https://github.com/rails/rails/blob/6-0-stable/actioncable/CHANGELOG.md
+[active-record]:  https://github.com/rails/rails/blob/6-0-stable/activerecord/CHANGELOG.md
+[active-storage]: https://github.com/rails/rails/blob/6-0-stable/activestorage/CHANGELOG.md
+[active-model]:   https://github.com/rails/rails/blob/6-0-stable/activemodel/CHANGELOG.md
+[active-support]: https://github.com/rails/rails/blob/6-0-stable/activesupport/CHANGELOG.md
+[active-job]:     https://github.com/rails/rails/blob/6-0-stable/activejob/CHANGELOG.md
+[guides]:         https://github.com/rails/rails/blob/6-0-stable/guides/CHANGELOG.md

guides/source/documents.yaml

@@ -201,6 +201,11 @@
       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 6.0 Release Notes
+      work_in_progress: true
+      url: 6_0_release_notes.html
+      description: Release notes for Rails 6.0.
     -
       name: Ruby on Rails 5.2 Release Notes
       url: 5_2_release_notes.html

guides/source/upgrading_ruby_on_rails.md

@@ -76,6 +76,8 @@ To allow you to upgrade to new defaults one by one, the update task has created
 Upgrading from Rails 5.2 to Rails 6.0
 -------------------------------------
 
+For more information on changes made to Rails 6.0 please see the [release notes](6_0_release_notes.html).
+
 ### Force SSL
 
 The `force_ssl` method on controllers has been deprecated and will be removed in