add slack badge to README
add slack badge to README

NB: not sure if this will make https unhappy due to mixed content security.
squeedee/concourse,concourse/concourse,zachgersh/concourse,sedouard/concourse,beni55/concourse,longnguyen11288/concourse,cjcjameson/concourse,beni55/concourse,longnguyen11288/concourse,squeedee/concourse,beni55/concourse,cjcjameson/concourse,concourse/concourse,CloudCredo/concourse,holgero/concourse,cjcjameson/concourse,sedouard/concourse,holgero/concourse,zachgersh/concourse,concourse/concourse,squeedee/concourse,concourse/concourse,sedouard/concourse,longnguyen11288/concourse,concourse/concourse,CloudCredo/concourse,zachgersh/concourse,holgero/concourse,CloudCredo/concourse