Add url property in the metadata

GNOME Shell emits a warning when "url" is absent in metadata.json,
adding it costs nothing. Links to the GIT repository because I
found no better page to reference.

extensions/example/metadata.json.in

@@ -3,5 +3,6 @@
  "name": "Hello, World!",
  "description": "An example extension to show how it works. Shows Hello, world when clicking on the top panel.",
  "shell-version": [ "@shell_current@" ],
- "localedir": "@LOCALEDIR@"
+ "localedir": "@LOCALEDIR@",
+ "url": "@url@"
 }