Adds comment to inform existing database tables will not be wiped

See https://jira.dosomething.org/browse/INF-41?jql=text%20~%20%22sql-sync%22

The `drush sql-sync` command does not remove additional tables in the local database as expected when using `ds  pull --stage`. 

This is a long standing issue with the `drush sql-sync` command. There has been an issue for several years to add a switch to drop the local unique database tables before sync.

https://github.com/drush-ops/drush/issues/161

This pull request adds the `--create-db` switch as it's expect that this switch will be the solution when this issue is resolved. The related comment the script echos has been updated to warn the user that local tables will have to be dropped rather than expecting the `ds` command to take care of the clean up.