Added bitbucket hook info and warning re: validating accounts.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Edits based on feedback from docs team.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Merge pull request #6967 from fredlf/copyedit-builds

Added bitbucket hook info and warning re: validating accounts.