Add guard

Guardfile

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+group :red_green_refactor, halt_on_fail: true do
+  guard :rspec, cmd: 'bundle exec rspec' do
+    watch(%r{^lib/(.+)\.rb$}) { |m| "spec/lib/#{m[1]}_spec.rb" }
+    watch(%r{^spec/.+_spec\.rb$})
+    watch('spec/spec_helper.rb') { 'spec' }
+  end
+
+  guard :rubocop do
+    watch(%r{(?:.+/)?\.rubocop(?:_todo)?\.yml$}) { |m| File.dirname(m[0]) }
+    watch('Gemfile')
+    watch('Guardfile')
+    watch('Rakefile')
+    watch(/.+\.gemspec$/)
+    watch(/.+\.rb$/)
+  end
+end