Docs: Extensions should not automatically alter the database structure.

See ckan/ideas#164.

doc/extensions/best-practices.rst

@@ -67,12 +67,28 @@ adding them to ``setup.py``, they should be added
 to ``requirements.txt``, which can be installed with::
 
   pip install -r requirements.txt
-  
+
 To prevent accidental breakage of your extension through backwards-incompatible 
 behaviour of newer versions of your dependencies, their versions should be pinned, 
 such as::
 
   requests==2.7.0
-  
+
 On the flip side, be mindful that this could also create version conflicts with
 requirements of considerably newer or older extensions.
+
+
+--------------------------------------------------
+Do not automatically modify the database structure
+--------------------------------------------------
+
+If your extension uses custom database tables then it needs to modify the
+database structure, for example to add the tables after its installation or to
+migrate them after an update. These modifications should not be performed
+automatically when the extension is loaded, since this can lead to `dead-locks
+and other problems`_.
+
+Instead, create a :doc:`paster command </maintaining/paster>` which can be run separately.
+
+.. _dead-locks and other problems: https://github.com/ckan/ideas-and-roadmap/issues/164
+