Add batch prediction example

[ZoranPandovski]

In all databases include examples for doing batch predictions.

[ZoranPandovski]

You can select some data from a similar table (or the same table) and get a bunch of predictions.

SELECT rental_price FROM mindsdb.rentals_predictor WHERE `select_data_query`='SELECT * FROM default.home_rentals WHERE days_on_market > 60';