contributing: instructions for local topgun
contributing: instructions for local topgun

with other folks trying to run the BOSH topgun suite locally, it's
useful to point them to how we (the concourse team) does it.

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

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