Add context to rakefile regarding testing strategy

Author: lizkenyon

Rakefile

@@ -7,7 +7,9 @@ require File.expand_path("../test/dummy/config/application", __FILE__)
 
 Rails.application.load_tasks
 
-# Clear the Rails-provided test task and define our own
+# Clear Rails' test task which expects bin/rails to exist.
+# This gem uses a dummy Rails app for testing but doesn't have bin/rails
+# since it's a gem, not a standalone Rails application.
 Rake::Task["test"].clear if Rake::Task.task_defined?("test")
 
 Rake::TestTask.new(:test) do |t|