Add a diagram of the project layout to guide new users.

[aambrioso1]

Beginning at the following paragraph the README shows new users how to build a minimal app that illustrates what is possible with Flask-Meld:

https://github.com/mikeabrahamsen/Flask-Meld/blob/main/README.md#initialize-meld-in-an-existing-project-alternative-method-to-get-started

A tree showing the project layout would be helpful.