Mention ZopeTxnExt and pyramid_tm in 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.
Merge pull request #749 from irskep/patch-1

Mention ZopeTransactionExtension and pyramid_tm in SQLAlchemy scaffold tutorial