Document new CF audit events

[gogolok]

New CF audit events have been added in the recent weeks.

https://github.com/cloudfoundry/cloud_controller_ng/pull/4733/changes

audit.stack.create
audit.stack.update
audit.stack.delete

https://github.com/cloudfoundry/cloud_controller_ng/pull/4740/changes

audit.organization_quota.create
audit.organization_quota.update
audit.organization_quota.delete
audit.organization_quota.apply
audit.space_quota.apply
audit.space_quota.create
audit.space_quota.delete
audit.space_quota.remove
audit.space_quota.update

https://github.com/cloudfoundry/cloud_controller_ng/pull/4741/changes

audit.buildpack.create
audit.buildpack.delete
audit.buildpack.update

https://github.com/cloudfoundry/cloud_controller_ng/pull/4791/changes

audit.buildpack.upload

https://github.com/cloudfoundry/cloud_controller_ng/pull/4843/changes

audit.app.build.staged
audit.app.build.failed