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 isn't perfectly sync'ed with 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.md you'll find there will help you get started.