Add USAGE banner to show-routes.

lib/pry-rails/railtie.rb

@@ -27,6 +27,12 @@ module PryRails
   Commands = Pry::CommandSet.new do
     create_command "show-routes", "Print out all defined routes in match order, with names." do
       def options(opt)
+        opt.banner unindent <<-USAGE
+          Usage: show-routes [-G]
+
+          show-routes displays the current Rails app's routes.
+        USAGE
+
         opt.on :G, "grep", "Filter output by regular expression", :argument => true
       end