Mention ZopeTransactionExtension and pyramid_tm in SQLAlchemy scaffold tutorial

If this page is found via web search while trying to configure SQLAlchemy from scratch without a scaffold, it is helpful to mention `pyramid_tm` as a requirement somewhere. Otherwise, a hapless user such as your humble author might spend lots of time wondering why his sessions aren't getting committed after his requests despite his best efforts to properly configure SQLAlchemy and `ZopeTransactionExtension`.

Possible further improvements:
* Dedicated section on configuring SQLAlchemy from scratch
* Add `ZopeTransactionExtension` and `pyramid_tm` to the glossary so they can be made linkable