Add build instructions for commercial licensees as well so that the
same README covers both opensource and enterprise users.

This removes the need having separate README file for commercial users.

Change-Id: Ibd3012b428c6fa85f47c1c459d8029ebfce92046
Reviewed-by: Jani Heikkinen <jani.heikkinen@theqtcompany.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>