providing examples for each CellType

[JHenneberg]

As promised in #17 I want to provide a few CellType examples. There are different options. The examples can live in the README.md as already started or a dedicated example folder could be created or actually both could be in sync.

I like the idea of having executable examples and simultaniously the same in the README.md. This of course means manually syncing of both because for now Markdown can not include any source code from a file automatically but those examples are probably not changed very often. Also some people might think the README.md is to bloaded then.