Add Barbican to the list of developer documentation

Barbican is a new incubator in Juno and the developer documentation
should be listed on http://docs.openstack.org/developer/.

Change-Id: I2a9eafad8b83a98ba60d00754dd280f97c59c8ea
Partial-Bug: #1343290
Merge "Add Barbican to the list of developer documentation"