Add locale file template

- Adds a more detailed template for the en.yml file provided with the extension skeleton.
- I've left the sample keys uncommented, but added a comment advising that they be removed.

framework/core/stubs/extension/locale/en.yml

@@ -1,2 +1,28 @@
 {{name}}:
-  # hello_world: "Hello, world!"
+
+  ##
+  # UNIQUE KEYS - The following keys are used in only one location each.
+  ##
+
+  # Strings in this namespace are used by the admin interface.
+  admin:
+
+    # These keys are provided as examples; please delete them before using.
+    sample_location:
+      sample_text: "Hello, world!"
+
+
+  # Strings in this namespace are used by the forum user interface.
+  forum:
+
+
+  # Strings in this namespace are used by the forum and admin interfaces.
+  lib:
+
+
+  ##
+  # REUSED STRINGS - The following keys are referenced by two or more unique keys.
+  ##
+
+  # Strings in this namespace are referenced by two or more unique keys.
+  ref: