#21 Give reason for functional groups in the user guide

[Jesko G. Lamm]

Add a statement to the user guide, explaining why there is the "functional group" block. An inspiration can be the following German statement: http://www.xing.com/net/gfse/funktionale-architekturen-fur-systeme-fas-688924/der-sinn-der-funktionalen-gruppe-in-der-fas-methode-40457002