Describe how to add extensions with `yo theia-extension` or otherwise

[scharf]

It would be great if there would be a brief description of how to add extensions with https://github.com/eclipse-theia/generator-theia-extension.

There appears to be some magic that is needed to get an extension running in this setup that is difficult to find out.

Alternatively, add all example extensions in a example-extensions directory with the instruction to simply copy the extension into the theia-extensions folder.