Tutorial for validation in devon4j

[GuentherJulian]

Create a tutorial that shows how to implement validation in devon4j.

Ideas:

1. Install the devonfw-ide
2. Create a Java project
3. Create an entity class
4. Create a REST service
5. Show how to implement validation within the REST service

Helpful links:
https://github.com/devonfw/devon4j/blob/master/documentation/guide-validation.asciidoc