Clarify how to include several RuboCop extensions

Author: pirj

docs/modules/ROOT/pages/usage.adoc

@@ -1,16 +1,24 @@
 = Usage
 
-You need to tell RuboCop to load the RSpec extension. There are three
-ways to do this:
+You need to tell RuboCop to load the RSpec extension.
+There are three ways to do this:
 
 == RuboCop configuration file
 
-Put this into your `.rubocop.yml`.
+Put this into your `.rubocop.yml`:
 
 ----
 require: rubocop-rspec
 ----
 
+or, if you are using several extensions:
+
+----
+require:
+  - rubocop-rspec
+  - rubocop-performance
+----
+
 Now you can run `rubocop` and it will automatically load the RuboCop RSpec
 cops together with the standard cops.