Add first draft of release notes for Rails 5.1 🎉

[ci skip]

guides/source/5_1_release_notes.md

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+**DO NOT READ THIS FILE ON GITHUB, GUIDES ARE PUBLISHED ON http://guides.rubyonrails.org.**
+
+Ruby on Rails 5.1 Release Notes
+===============================
+
+Highlights in Rails 5.1:
+
+* Yarn Support
+* Optional Webpack support
+* jQuery no longer a default dependency
+* System tests
+* Encrypted secrets
+* Parameterized mailers
+* Direct & resolved routes
+* Unification of form_for and form_tag into form_with
+
+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/5-1-stable) in the main Rails
+repository on GitHub.
+
+--------------------------------------------------------------------------------
+
+Upgrading to Rails 5.1
+----------------------
+
+ToDo
+
+Major Features
+--------------
+
+### Yarn Support
+
+### System tests
+
+### Encrypted secrets
+
+### Parameterized mailers
+
+### Direct & resolved routes
+
+Railties
+--------
+
+Please refer to the [Changelog][railties] for detailed changes.
+
+Action Pack
+-----------
+
+Please refer to the [Changelog][action-pack] for detailed changes.
+
+Action View
+-------------
+
+Please refer to the [Changelog][action-view] for detailed changes.
+
+Action Mailer
+-------------
+
+Please refer to the [Changelog][action-mailer] for detailed changes.
+
+Active Record
+-------------
+
+Please refer to the [Changelog][active-record] for detailed changes.
+
+Active Model
+------------
+
+Please refer to the [Changelog][active-model] for detailed changes.
+
+Active Job
+-----------
+
+Please refer to the [Changelog][active-job] for detailed changes.
+
+Active Support
+--------------
+
+Please refer to the [Changelog][active-support] for detailed 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/5-1-stable/railties/CHANGELOG.md
+[action-pack]:    https://github.com/rails/rails/blob/5-1-stable/actionpack/CHANGELOG.md
+[action-view]:    https://github.com/rails/rails/blob/5-1-stable/actionview/CHANGELOG.md
+[action-mailer]:  https://github.com/rails/rails/blob/5-1-stable/actionmailer/CHANGELOG.md
+[action-cable]:   https://github.com/rails/rails/blob/5-1-stable/actioncable/CHANGELOG.md
+[active-record]:  https://github.com/rails/rails/blob/5-1-stable/activerecord/CHANGELOG.md
+[active-model]:   https://github.com/rails/rails/blob/5-1-stable/activemodel/CHANGELOG.md
+[active-support]: https://github.com/rails/rails/blob/5-1-stable/activesupport/CHANGELOG.md
+[active-job]:     https://github.com/rails/rails/blob/5-1-stable/activejob/CHANGELOG.md