Adding extension to release created in the tool

When you work with an existing Process then that process might already have extensions, and they should be kept (as you can't remove an extension half way through an contracting process)

The tool should add existing extensions by default when addemptyrelease (think it already does)?

But what if you want to start using another extension?

Either

• the tool could have an option for this to make it easier
• we should document how to do this

Currently, after running addemptyrelease you would just edit the package.json (I think)