Fixed #21. Document a known issue with bulk deletion under Django 1.1, this is…

Fixed #21.  Document a known issue with bulk deletion under Django 1.1, this is why 1.2 is awesome ;)

docs/index.txt

@@ -10,6 +10,7 @@ tagging to your project easy and fun.
    getting_started
    api
    custom_through
+   issues
 
 Indices and tables
 ==================

docs/issues.txt

+Known Issues
+============
+
+Currently there is 1 known issue:
+
+ * When run under Django 1.1, doing ``Model.objects.all().delete()`` (or any
+   bulk deletion operation) on a model with a ``TaggableManager`` will result
+   in losing the tags for items beyond just those assosciated with the deleted
+   objects.  This issue is not present in Django 1.2.