Added warnings in the documentation when subclassing MP_Node and adding a new Meta.
Added warnings in the documentation when subclassing MP_Node and adding a new Meta.

Fixes issue 5

skirsdeda/django-treebeard,skirsdeda/django-treebeard,tabo/django-treebeard,tabo/django-treebeard,django-treebeard/django-treebeard,skirsdeda/django-treebeard,tabo/django-treebeard,skirsdeda/django-treebeard,django-treebeard/django-treebeard,tabo/django-treebeard,django-treebeard/django-treebeard