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 <[e-mail]>
Reviewed-by: Ido Schimmel <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

tools/testing/selftests/net/forwarding/README

···
 
 o Where possible, reuse an existing topology for different tests instead
   of recreating the same topology.
+o Tests that use anything but the most trivial topologies should include
+  an ASCII art showing the topology.
 o Where possible, IPv6 and IPv4 addresses shall conform to RFC 3849 and
   RFC 5737, respectively.
 o Where possible, tests shall be written so that they can be reused by