contributing: instructions for -nodes=N
contributing: instructions for -nodes=N

k8s-topgun tests are much better run concurrently rather than serially
given how slow the initialization times are compared to unit tests.

Signed-off-by: Ciro S. Costa <4f5054ab2db051d03adf9068233a850c82b24437@pivotal.io>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse