x86/msr: Add a pointer to an URL which contains further details

After having collected the majority of reports about MSRs being written
by userspace tools and what tools those are, and all newer reports
mostly repeating, add an URL where detailed information is gathered and
kept up-to-date.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20201205002825.19107-1-bp@alien8.de