For a general introduction to XOS and how it is used in CORD, see http://guide.xosproject.org. The "Developer Guide" at that URL is especially helpful, although it is sync'ed with the previous release (Burwell), which lags what's in master. Additional design notes, presentations, and other collateral are also available at http://xosproject.org and http://cord.onosproject.org.
One quick way to get started is to build and run the containers in containers (see the README in that directory for more information).
Another quick way to get started is to look at the collection of canned configurations in xos/configurations. The cord configuration in that directory corresponds to our current CORD development environment, and the README you'll find there will help you get started.