Nexus deployment credentials in settings.xml

This challenge is about packing and explaining why you should not have nexus deployment credentials in your github project hardcoded. However, we will not have an actual nexus setup right now.

TODO's:

• - Create a settings.xml for Maven to be able to connect to an imaginary Nexus repo See https://github.com/sonatype/nexus-book-examples/blob/master/maven/settings/settings.xml as an example.
• - Implement a challenge that reads the actual challenge secret value from the settings.xml (see contributing.md for more details)