selftests: forwarding: README: Require diagrams

ASCII art diagrams are well suited for presenting the topology that a
test uses while being easy to embed directly in the test file iteslf.
They make the information very easy to grasp even for simple topologies,
and for more complex ones they are almost essential, as figuring out the
interconnects from the script itself proves to be difficult.

Therefore state the requirement for topology ASCII art in README.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>