Update README and add contrib dir

README.md

@@ -31,6 +31,8 @@ cd <path to your ns-3 installation>
 ./test.py --suite ethernet --text test-logs
 ./test.py --example ethernet-point2point-withPropagationDelay
 ```
+Note that with version 3.40 of ns-3, the “neighbor-cache” test fails without any connection or consequence to our library. However, the “mobility-trace,” “attributes,” “data-rate,” and “time” tests fail because of our library. Indeed, in order to simulate communications at over 1Gb/s, the “ethernet-test-suite.cc” test sets the time resolution to picoseconds instead of nanoseconds, thus causing side effects in other tests. Line 996 (i.e. “Time::SetResolution(Time::PS);”) can be commented out to resolve this issue.
+
 
 To run an example (e.g. "ethernet-point2point.cc") :
 ```console