Changed the note about the issues with using MP_Node.fix_tree() with FKs into a warning.
Changed the note about the issues with using MP_Node.fix_tree() with FKs into a warning.

Fixes issue 13.

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