Add to documentation an example on how to use reflection

[vak]

Hi,

i'm trying to use GeoAlchemy2 and saw in docs that DB reflection is supported. But there was no example on that :( The only thing found is tests/functional.py but it gave me no clue.

The following code snippet:

from sqlalchemy import create_engine, MetaData, Table
sql_uri = "postgresql+psycopg2://myuser:mypass@mydb.host.tld/mydbname" 
engine  = create_engine(sql_uri)
meta.reflect(bind=engine)

shows that geometry is not recognized:

/usr/lib/python2.7/site-packages/SQLAlchemy-1.0.8-py2.7-linux-x86_64.egg/sqlalchemy/dialects/postgresql/base.py:2454: SAWarning: Did not recognize type 'geometry' of column 'geometry'
/usr/lib/python2.7/site-packages/SQLAlchemy-1.0.8-py2.7-linux-x86_64.egg/sqlalchemy/dialects/postgresql/base.py:2454: SAWarning: Did not recognize type 'geometry' of column 'point'
...

which is fully clear, because one has to provide somehow the stuff from GeoAlchemy2.

You may want to add in docs that simple import like this is the only thing needed for the above SQLAlchemy reflection code snippet:

from geoalchemy2 import Geometry, Geography