rr_graph: Adding warning about edges_for_nodes being slow.
rr_graph: Adding warning about edges_for_nodes being slow.

Signed-off-by: Tim 'mithro' Ansell <b1c1d8736f20db3fb6c1c66bb1455ed43909f0d8@mith.ro>

SymbiFlow/symbiflow-arch-defs,SymbiFlow/symbiflow-arch-defs