Changed name of XmlReader class to MyXmlReader

to avoid collision with PHP's XMLReader class:

Argument 2 passed to Configure::config() must be an instance of ConfigReaderInterface, instance of XMLReader given.
Added description of the problem
Added warning box
Adhereing to PR #5230

Harden interface for Config engine classes.
Added external link

fixed line length
Changed external link to en
Merge pull request #2033 from hmic/patch-4

Changed name of XmlReader class to MyXmlReader