add "yard" rake task

Rakefile

@@ -8,9 +8,14 @@ end
 require "bundler/gem_tasks"
 require "rspec/core/rake_task"
 require "rake/clean"
+require "yard"
 
 CLEAN.include 'build_tests_run'
 
 RSpec::Core::RakeTask.new(:spec)
 
+YARD::Rake::YardocTask.new do |yard|
+  yard.files = ['lib/**/*.rb']
+end
+
 task :default => :spec