Explain DownloadableArchives tag in documentation.
Explain DownloadableArchives tag in documentation.

This fixes https://bugreports.qt-project.org/browse/QTIFW-113

Change-Id: I8bf2c89bd1007a9576d8e4d148b050f9a1ac3925
Reviewed-by: Leena Miettinen <885fda02d52616fec24ed5fed9b3f0568fce3ead@nokia.com>

olear/qtifw,olear/qtifw,olear/qtifw,olear/qtifw