Add explicit documentation for adding modules to existing projects

[danielcompton]

The docs on https://cursiveclojure.com/userguide/leiningen.html are pretty good for multi module projects, but it could be good to expand on this point:

or using the context menu in the project tool window.

It's not explicitly clear that you need to use "New > Module" from either File or the Context menu to accomplish this. It could be changed to something like this but less wordy:

You can then add new modules to the project using either the + button in the Leiningen tool window, or by "New > Module" from the context menu in the project tool window or the File menu. (include screenshot)