Merge pull request #28709 from prathamesh-sonpatki/release-notes-1

Started adding release notes for Rails 5.1 [ci skip]

guides/source/5_1_release_notes.md

@@ -228,90 +228,140 @@ can generate form tags based on URLs, scopes or models.
 Railties
 --------
 
+### Removals
+
+### Deprecations
+
+### Notable Changes
+
 Please refer to the [Changelog][railties] for detailed changes.
 
+Action Cable
+-----------
+
 ### Removals
 
 ### Deprecations
 
-### Notable changes
+### Notable Changes
+
+*   Added support for `channel_prefix` to Redis and evented Redis adapters
+    in `cable.yml` to avoid name collisions when using the same Redis server
+    with multiple applications.
+    ([Pull Request](https://github.com/rails/rails/pull/27425))
+
+Please refer to the [Changelog][action-cable] for detailed changes.
 
 Action Pack
 -----------
 
-Please refer to the [Changelog][action-pack] for detailed changes.
-
 ### Removals
 
 ### Deprecations
 
-### Notable changes
+### Notable Changes
+
+Please refer to the [Changelog][action-pack] for detailed changes.
 
 Action View
 -------------
 
-Please refer to the [Changelog][action-view] for detailed changes.
-
 ### Removals
 
 ### Deprecations
 
-### Notable changes
+### Notable Changes
+
+Please refer to the [Changelog][action-view] for detailed changes.
 
 Action Mailer
 -------------
 
-Please refer to the [Changelog][action-mailer] for detailed changes.
-
 ### Removals
 
 ### Deprecations
 
-### Notable changes
+### Notable Changes
+
+*   Allowed setting custom content type when attachments are included
+    and body is set inline.
+    ([Pull Request](https://github.com/rails/rails/pull/27227))
+
+*   Allowed passing lambdas as values to the `default` method.
+    ([Commit](https://github.com/rails/rails/commit/1cec84ad2ddd843484ed40b1eb7492063ce71baf))
+
+*   Added support for parameterized invocation of mailers to share before filters and defaults
+    between different mailer actions.
+    ([Commit](https://github.com/rails/rails/commit/1cec84ad2ddd843484ed40b1eb7492063ce71baf))
+
+*   Passed the incoming arguments to the mailer action to `process.action_mailer` event under
+    an `args` key.
+    ([Pull Request](https://github.com/rails/rails/pull/27900))
+
+Please refer to the [Changelog][action-mailer] for detailed changes.
 
 Active Record
 -------------
 
-Please refer to the [Changelog][active-record] for detailed changes.
-
 ### Removals
 
 ### Deprecations
 
-### Notable changes
+### Notable Changes
+
+*   Skipped comments in the output of `mysqldump` command by default.
+    ([Pull Request](https://github.com/rails/rails/pull/23301))
+
+*   Fixed `ActiveRecord::Relation#count` to use Ruby's `Enumerable#count` for counting
+    records when a block is passed as argument instead of silently ignoring the
+    passed block.
+    ([Pull Request](https://github.com/rails/rails/pull/24203))
+
+*   Pass `"-v ON_ERROR_STOP=1"` flag with `psql` command to not suppress SQL errors.
+    ([Pull Request](https://github.com/rails/rails/pull/24773))
+
+Please refer to the [Changelog][active-record] for detailed changes.
 
 Active Model
 ------------
 
-Please refer to the [Changelog][active-model] for detailed changes.
-
 ### Removals
 
 ### Deprecations
 
-### Notable changes
+### Notable Changes
+
+Please refer to the [Changelog][active-model] for detailed changes.
 
 Active Job
 -----------
 
-Please refer to the [Changelog][active-job] for detailed changes.
-
 ### Removals
 
 ### Deprecations
 
-### Notable changes
+### Notable Changes
+
+Please refer to the [Changelog][active-job] for detailed changes.
 
 Active Support
 --------------
 
-Please refer to the [Changelog][active-support] for detailed changes.
-
 ### Removals
 
 ### Deprecations
 
-### Notable changes
+### Notable Changes
+
+*   Added `Module#delegate_missing_to` to delegate method calls not
+    defined for the current object to a proxy object.
+    ([Pull Request](https://github.com/rails/rails/pull/23930))
+
+*   Added `Date#all_day` which returns a range representing the whole day
+    of the current date & time.
+    ([Pull Request](https://github.com/rails/rails/pull/24930))
+
+Please refer to the [Changelog][active-support] for detailed changes.
 
 Credits
 -------