Document .app_rules template

.gitlab-ci.yml

@@ -42,6 +42,17 @@
 # ============
 #
 
+# the .app_rules should be used whenever an app-specific job is executed.
+# just add the variable app to the job like this:
+
+#   variables:
+#     APP: "eventrouter"
+
+# and import the template with
+
+#   extends: .app_rules
+
+# .app_rules will ensure that the job is only executed when files related to the app changed in the repo
 .app_rules:
   before_script:
     - *debug_information