Add a note on status of query generation

Author: jingw

README.rst

@@ -1,3 +1,4 @@
+======
 PyHive
 ======
 
@@ -31,6 +32,9 @@ First install this package to register it with SQLAlchemy (see ``setup.py``).
     logs = Table('my_awesome_data', MetaData(bind=engine), autoload=True)
     print select([func.count('*')], from_obj=logs).scalar()
 
+Note: query generation functionality is not exhaustive or fully tested, but there should be no
+problem with raw SQL.
+
 Requirements
 ============