README: Document that tools and libraries should be upgraded together.

README: Document that tools and libraries should be upgraded together.

On some distributions it was possible to upgrade separate parts of elfutils
individually. That causes hard to debug issues since some tools (e.g. readelf)
and some libraries (e.g. libdw) depend on libelf internal data structures,
which could be different between versions (or when configured with different
flags). So explicitly document that they should always be upgraded together.

Signed-off-by: Mark Wielaard <mjw@redhat.com>