change markdown links to reference style
change markdown links to reference style

Signed-off-by: Jeanie Jung <8425aff5642aab6c6806ce8ad58414b419f23fe1@pivotal.io>

CloudCredo/concourse,holgero/concourse,squeedee/concourse,CloudCredo/concourse,zachgersh/concourse,longnguyen11288/concourse,beni55/concourse,concourse/concourse,longnguyen11288/concourse,concourse/concourse,holgero/concourse,cjcjameson/concourse,holgero/concourse,squeedee/concourse,beni55/concourse,zachgersh/concourse,sedouard/concourse,cjcjameson/concourse,sedouard/concourse,beni55/concourse,concourse/concourse,cjcjameson/concourse,zachgersh/concourse,concourse/concourse,longnguyen11288/concourse,CloudCredo/concourse,concourse/concourse,sedouard/concourse,squeedee/concourse