Add Codenvy factory to test this example inside Codenvy

[benoitf]

Codenvy Workspace Developer

Hello,

this pull request introduces a new _“Codenvy workspace developer”_ button in the README of the project.

This button enables anyone to test or contribute to the project without installing anything.

When a developer presses the button, a new workspace is created:

• A project runtime is created in a Docker container running in the Codenvy cloud (for free). You can customize this container using a Dockerfile.
• Source code is automatically cloned into the workspace as a “local” copy.
• A command set is added so users can build / run / debug the project with a single click.
• The project is automatically run so a new user can experience it hassle-free.
• A pull request panel in the IDE simplifies the process of creating a PR for new users. Experienced users can use the git menu in the IDE, or issue git commands from the built-in terminal.

This does not require _any changes_ to the source code and has _no cost_ to you or your users.

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This change is 

[larrytalley]

benoitf, thanks so much for suggesting this! I'm not involved in this project, but, I'm now going to make use of your Codenvy factory to jump start my exploration of Vaadin. Of course I'll have to adapt your URL to recent changes, as follows: https://codenvy.io/f?id=ej6koycnt4maubns