Document common errors in a Troubleshooting section

PostgreSQL integration, e.g. https://github.com/onyxfish/csvkit/issues/522 and linked issues

You don't appear to have the necessary database backend installed for connection string you're trying to use. Available backends include:

Postgresql: pip install psycopg2
MySQL:      pip install MySQL-python

For details on connection strings and other backends, please see the SQLAlchemy documentation on dialects at: 

http://www.sqlalchemy.org/docs/dialects/

Sniff-limit and type inference errors, e.g. from https://github.com/onyxfish/csvkit/issues/166

Should we add a troubleshooting section with something along the lines of "try it again with --snifflimit 0"? Another common one is "try it again with --no-inference".