package/perl-libwww: Fix legal-info

Fixes
http://autobuild.buildroot.net/results/6c6/6c6fb6001101ff528176b887ed8c95ef49313afb/
http://autobuild.buildroot.net/results/56d/56d42e7691d4eb1a682c49c0fa6c951d0e3069ec/



[Thomas: use the README file, since it contains a copyright notice and
the name of the license, instead of not having any license file at
all.]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>