Update property files: add header note

Signed-off-by: Pekka Helenius <fincer89@hotmail.com>

bookstore/src/main/resources/messages.properties

@@ -1,5 +1,15 @@
 # Pekka Helenius <fincer89@hotmail.com>, Fjordtek 2020
 
+# NOTE:
+#   These properties represent front-end messages, i.e. what we
+#   show users in the front-end.
+#
+#   These keys are not back-end properties, so never refer 
+#   them in Spring Environment context.
+#   (for instance, you can't use getProperty method for these keys)
+
+
+
 # Message keys with values.
 # Prepare environment for possible multi-language support
 

bookstore/src/main/resources/website.properties

@@ -1,5 +1,13 @@
 # Pekka Helenius <fincer89@hotmail.com>, Fjordtek 2020
 
+# NOTE:
+#   These properties represent internal application properties,
+#   NOT linguistic front-end message strings. Never refer to keys
+#   in this file in your application front-end messages.
+#   These keys are accessible in Spring Environment context.
+
+
+
 # BookController
 # Public end points
 #