RUBY-2411 Port query cache middleware to driver

 (#2165)

* RUBY-2411 Port query cache middleware to driver

* fix typos

Co-authored-by: Oleg Pudeyev <[email protected]>

Author: p-mongo

source/tutorials/query-cache.txt

@@ -20,8 +20,10 @@ the cached reuslts to prevent unnecessary roundtrips to the database.
 Usage
 =====
 
-The query cache is disabled by default. It cache can be enabled on the global
-scope as well as within the context of a specific block.
+The query cache is disabled by default. It can be enabled on the global
+scope as well as within the context of a specific block. The driver also
+provides a :ref:`Rack middleware <query-cache-middleware>` to enable the
+query cache automatically for each web request.
 
 To enable the query cache globally:
 
@@ -231,3 +233,24 @@ You may read more about system collections in the
   Even when the query cache is enabled, query results from system collections
   will not be cached.
 
+
+.. _query-cache-middleware:
+
+Query Cache Middleware
+======================
+
+The driver provides a Rack middleware which enables the query cache for the
+duration of each web request. Below is an example of how to enable the
+query cache middleware in a Ruby on Rails application:
+
+.. code-block:: ruby
+
+  # config/application.rb
+
+  # Add Mongo::QueryCache::Middleware at the bottom of the middleware stack
+  # or before other middleware that queries MongoDB.
+  config.middleware.use Mongo::QueryCache::Middleware
+
+Please refer to the `Rails on Rack guide
+<https://guides.rubyonrails.org/rails_on_rack.html#configuring-middleware-stack>`_
+for more information about using Rack middleware in Rails applications.