Adding User and Developer documentation

To make it easier to start developing for this project I've added several use cases. The content is more or less extracted from the code.

Several use cases, need improvement, as a description about the 'attack' command (is not yet there, but expecting something like that as the player can equip weapon and armour).

Also added a Maven build file 'pom.xml' which can render all the document into a HTML site.
Maven needs to be installen. After that use the command:  `mvn site:site`