Updated secrets.yml template to use YAML anchor/reference syntax

 - This is done to show how secrets or configuration settings can be
   shared in multiple environments similar to default database.yml file.

railties/lib/rails/generators/rails/app/templates/config/secrets.yml

@@ -10,6 +10,21 @@
 # Make sure the secrets in this file are kept private
 # if you're sharing your code publicly.
 
+# You can also share common secrets in multiple environments
+# using YAML anchor/reference syntax.
+
+# common: &common
+#   api_secret: api_secret
+
+# development:
+#   <<: *common
+#
+# test:
+#   <<: *common
+#
+# production:
+#  api_secret: <%= ENV['API_SECRET'] %>
+
 development:
   secret_key_base: <%= app_secret %>