QCollator: Add note about ICU dependency.

Task-number: QTBUG-28766
Change-Id: If661915457c4874a72e1111b85897aea596362bf
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>