update error message for un-upgraded external stores.

Author: dimitri-yatsenko

datajoint/heading.py

@@ -247,8 +247,10 @@ def init_from_database(self, conn, database, table_name, context):
                     category = next(c for c in SPECIAL_TYPES if TYPE_PATTERN[c].match(attr['type']))
                 except StopIteration:
                     if attr['type'].startswith('external'):
-                        raise DataJointError('Legacy datatype `{type}`. '
-                                             'Migrate your external stores to datajoint 0.12'.format(**attr)) from None
+                        url = "https://docs.datajoint.io/python/admin/5-blob-config.html" \
+                              "#migration-between-datajoint-v0-11-and-v0-12"
+                        raise DataJointError('Legacy datatype `{type}`. Migrate your external stores to '
+                                             'datajoint 0.12: {url}'.format(url=url, **attr)) from None
                     raise DataJointError('Unknown attribute type `{type}`'.format(**attr)) from None
                 if category == 'FILEPATH' and not _support_filepath_types():
                     raise DataJointError("""