RUBY-1271 Change Streams tutorial

Author: estolfo

docs/ruby-driver-tutorials.txt

@@ -20,6 +20,7 @@ operations available in the Ruby driver.
    /tutorials/ruby-driver-collection-tasks
    /tutorials/ruby-driver-projections
    /tutorials/ruby-driver-sessions
+   /tutorials/ruby-driver-change-streams
    /tutorials/ruby-driver-admin-tasks
    /tutorials/ruby-driver-indexing
    /tutorials/ruby-driver-collations

docs/tutorials/ruby-driver-change-streams.txt

@@ -0,0 +1,75 @@
+==============
+Change Streams
+==============
+
+.. default-domain:: mongodb
+
+.. contents:: On this page
+   :local:
+   :backlinks: none
+   :depth: 1
+   :class: singlecol
+
+As of version 3.6 of the MongoDB server, a new ``$changeStream`` pipeline stage is supported in the aggregation
+framework. The Ruby driver provides an API for receiving notifications for changes to a particular collection using this
+new pipeline stage. Although you can create a change stream using the pipeline operator and aggregation framework
+directly, it is recommended to use the driver API described below as the driver resumes the change stream if there is
+timeout or network error.
+
+Change streams on the server requires a ``"majority"`` read concern or no read concern.
+
+Change streams do not work properly with JRuby because of the issue documented here_.
+
+.. _here: https://github.com/jruby/jruby/issues/4212
+
+Namely, JRuby eagerly evaluates ```#next`` on an Enumerator in a background green thread.
+So calling ```#next`` on the change stream will cause getmores to be called in a loop in the background.
+
+Watching for changes on a particular collection
+-----------------------------------------------
+
+A change stream is created by calling the ``#watch`` method on a collection:
+
+.. code-block:: ruby
+
+  stream = collection.watch
+  collection.insert_one(a: 1)
+  doc = stream.to_enum.next
+  process(doc)
+
+
+You can also receive the notifications as they are available:
+
+.. code-block:: ruby
+
+  stream = client[:test].watch
+  enum = stream.to_enum
+  while doc = enum.next
+    process(doc)
+  end
+
+
+The change stream can take filters in the aggregation framework pipeline operator format:
+
+.. code-block:: ruby
+
+  stream = collection.watch([{'$match' => { 'operationType' => {'$in' => ['insert', 'replace'] } } },
+                             {'$match' => { 'fullDocument.n' => { '$gte' => 1 } } }
+                            ])
+  enum = stream.to_enum
+  while doc = enum.next
+    process(doc)
+  end
+
+Close a Change Stream
+---------------------
+
+You can close a Change Stream by calling the ``#close`` method:
+
+.. code-block:: ruby
+
+  stream = collection.watch
+  collection.insert_one(a: 1)
+  doc = stream.to_enum.next
+  process(doc)
+  stream.close