Added warning about theme availability after generation

When I followed this tutorial I expected the theme to show in the admin, but it didn't. It was only after executing the step further on that it was displayed.

Also, I ran the command before creating any templates, and I had an XML validation error about missing `<service/>` elements from `<services/>`
Merge pull request #19 from dantleech/patch-1

Added warning about theme availability after generation