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.

[JonasHelming]

see eclipse-theia/theia-website#622
What you need to do:

Add you extension to the product (e.g. https://github.com/eclipse-theia/theia-blueprint/blob/master/applications/browser/package.json for browser ans similarily for Electron)

[JonasHelming]

(but we will document this on the website)