Add 7th edition link

Author: mhartl

Guardfile

@@ -1,3 +1,7 @@
+# This is the Guardfile for the 6th edition of the Rails Tutorial.
+# For the 7th edition, see here:
+# https://github.com/learnenough/rails_tutorial_sample_app_7th_ed/blob/main/Guardfile
+
 require 'active_support/core_ext/string'
 # Defines the matching rules for Guard.
 guard :minitest, spring: "bin/rails test", all_on_start: false do
@@ -69,4 +73,4 @@ end
 # Returns all tests for the given resource.
 def resource_tests(resource)
   integration_tests(resource) << controller_test(resource)
-end
+end