README: Don't advocate configure option -nokia-developer

Advocating -nokia-developer for any 'developer-build' sounds wrong.
Instead use -developer-build .

Change-Id: I8b93bc1222fd2754f8ed120d3bd856a4cd38f88c
Sanity-Review: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Sergio Ahumada <sergio.ahumada@nokia.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>