Add instructions how to package test UIs in to a war

[Wnt]

At least I often use the development time testing UIs in an online demo I link to in the Vaadin directory add-on page. It is not clear for me how I could package the BasicMyComponentUsageUI etc. in to a war I could deploy into e.g. Jelastic.

[mstahv]

One can use plain UI classes as well and create a separate war project for demos. This should definitely be discussed in the README file generated for the project.

[mstahv]

Also to the non-gwt version.