Added bitbucket hook info and warning re: validating accounts.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)

(Closes https://github.com/dotcloud/docker-index/issues/1313)
Both GH and BB use repo and repository interchangeably. That also seems to be the common parlance amongst devs and ops, at least here in 'murica. I'm open to standardizing on one or the other if others deem it necessary, but I don't think it is.