add informations how to create a custom doctrine mapping

i tryed to explain a bit how doctrine tries to find the correct mapping, the driver and how you can configure your own mapping.
i really like this. the main concern of this PR is just to document how to create a custom mapping.
i already documented the ``type`` and the ``dir``. we sould go on with is_bundle, prefix and alias - but i would like to keep the PR as small as possible to provide a documentation about how to use custom mapping paths as fast as possible.