Instruct new users about rails-controller-testing

[ci skip]

README.md

@@ -110,7 +110,7 @@ Use this branch at your discretion!
 
 ### RSpec
 
-Include `shoulda-matchers` in your Gemfile:
+Start by including `shoulda-matchers` in your Gemfile:
 
 ``` ruby
 group :test do
@@ -118,6 +118,16 @@ group :test do
 end
 ```
 
+We typically use `rspec-rails` alongside `shoulda-matchers`, but if for some
+reason you do not want to do this, and your app is on Rails 5+, you'll want to
+add the `rails-controller-testing` gem as well:
+
+``` ruby
+group :test do
+  gem 'rails-controller-testing'
+end
+```
+
 Now you need to tell the gem a couple of things:
 
 * Which test framework you're using