the extension file has to have also directive elements (skipper-configuration, orm-configuration,... ) to be able to parse it correctly (and because it's necessary to tell Skipper for which ORM framework you're extending it).
This means something like this:
<template element='entity' use-case='create' selector='new-project'>...
The best way how to start with templates is to edit existing Skipper configuration file. And as soon as you are satisfied with customized templates, create custom file like you're mentioning.