Fix XML format documentation

[plata]

In general, the documentation in XMLPG.html is not really accessible. I would prefer to have it in the README.

• document all possibilities of the XML format (e.g. defaultValue is missing)
• fix documentation (e.g. fixed → fixedlist, variable → variablelist)

What makes it even worse: The DIS2012.xml file is not accepted by XMLPG.

[leif81]

Thanks @plata

This was partially fixed in #18 where it was converted to markdown XMLPG.md

Please feel free to submit a PR for the other issues with the docs you observed. I'm not sure I follow.