* Specification:
  * Each plug-in module should declare one or more extensions. They add functionality to extension points defined in other modules.
  * Plug-ins may also declare extension points. These are places that allow adding functionality to the developed plug-in. 
  * Plug-ins must have:
   * source code
   * tests( unit, integration)
   * configuration files
   * build files
   * compiled code. 
  * They may also have:
   * resources
   * external libraries.