Be more explicit on where qt licenses are stored

Mention that the Qt Licenses are nowadays usually fetched automatically.
Anyhow, if this goes wrong be more explicit on where the files are
searched and how to work around things.

Change-Id: I9f58f99170f85ba236d7f28f18d51c77d23d1956
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>