Update the instructions to describe the unified Qt
installer and mention the Qt Account.

Change-Id: Id21179558e2e6366366d6c9a0b094eba14b87ef3
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>