Add arquillian.xml configuration section to the documentation

At the moment there is no chapter in the documentation which describes the different workings of arquillian.xml. AFAIU each container adapter can have its own properties, but I think there is still a need to describe the basic workings, eg how to select a named container configuration via arquillian.launch.

See also:

• http://stackoverflow.com/questions/9740938/how-to-select-a-container-by-qualifyer-from-arquillian-xml
• http://stackoverflow.com/questions/10119544/where-can-i-find-the-arquillian-xml-configuration-documentation

Given that arquillian.xml is the main configuration file it is only reasonable that people try to understand Arquillian from this angle.