mention include_columns in docs; see #561
mention include_columns in docs; see #561

(sorry Mike, I did have a unit test but didn't commit it somehow)

git-svn-id: 655ff90ec95d1eeadb1ee4bb9db742a3c015d499@3080 8cd8332f-0806-0410-a4b6-96f4b9520244

obeattie/sqlalchemy,obeattie/sqlalchemy,obeattie/sqlalchemy