[deploy-sourcegraph] write documentation to add password for Postgres user

Created by: ggilmore

Following up on https://github.com/sourcegraph/sourcegraph/issues/2192#issuecomment-461421175

https://github.com/sourcegraph/deploy-sourcegraph/blob/master/docs/configure.md should have instructions about setting up a password for the 'sg' Postgres user. You'll probably need to:

1. modify https://github.com/sourcegraph/deploy-sourcegraph/blob/master/base/pgsql/pgsql.Deployment.yaml
2. modify https://github.com/sourcegraph/deploy-sourcegraph/tree/master/base/frontend
3. modify https://github.com/sourcegraph/deploy-sourcegraph/blob/master/base/management-console/management-console.Deployment.yaml
4. add instructions about how to modify an existing Postgres instance from having a passwordless user to user with a password

https://github.com/sourcegraph/deploy-sourcegraph-docker will probably need similar instructions cc @slimsag