Update README for 3.1.

Update some links, try to make it less Qt-4-specific.
Add information about post-mortem debugging.

Change-Id: I1be1dc73529dad0534ad7456956882405d6dba47
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Robert Loehning <robert.loehning@digia.com>