Document that snmp package is needed for running tests

[johannaengland]

When I tried running the tests on a new Ubuntu installation the tests test_when_link_down_is_received_it_should_create_portstate_event, test_when_port_does_not_match_watch_pattern_it_should_ignore_link_traps and test_when_port_matches_ignore_pattern_it_should_ignore_link_traps from tests/trapobservers/link_traps_test.py failed with the following error

AssertionError: snmptrap command exited with error
assert 127 == 0
  + where 127 = <Process 47872>.returncode

I figured out that that was due to not having the snmp package installed.
It is easily fixed on Ubuntu using apt install snmp.

This should be documented in the running tests section of the README.
I don't know how this affects other OS'.