Update READMEs for BigQuery

[garrettjonesgoogle]

• Main README: move to GA section
• BigQuery README: remove warning about being work-in-progress

and update anything else that is amiss...

[marcin-kolda]

Please also update version on https://cloud.google.com/bigquery/docs/reference/libraries#client-libraries-install-java as it's confusing.

[pongad]

@jabubake Could you tell me how to update the site? Feel free to assign it back to me if you want.

[jabubake]

@pongad Ack, submitted request internally to update version (cc-ed you on it). It would be good to have a standing instruction on updating the docs every time a release is cut for the library.

[pongad]

Everything is up to date now.