Add README to composer project templates

Problem/Motivation

Composer templates do not have README documentation.

Proposed resolution

Add README documentation.

Each template directory should have its own README.txt file, which gives an explanation of how to use the project, and also links to documentation.

The goal is that the end-user has this README file available after they have issued a composer create-project command.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet