document how to use Octave graphical features

May want to document how to start a container with the ability to connect to the host's X11 socket, over either TCP or UNIX. Should mention how to mount the UNIX socket directory, and should show a working example of either xauth or xhost or both for granting permissions.

Maybe in a new top-level section in README.md, or maybe in a completely new file.

Migrated from octave-snapshot#6 (closed).