add scripts/create-migration to contributing docs
add scripts/create-migration to contributing docs

Signed-off-by: Jamie Klassen <d740210502544aa0776b92978c6db85774130b86@pivotal.io>
Co-authored-by: Clara Fu <d5c8bdbc15689555c92563892e959c79b3fc425f@pivotal.io>

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