Have a look at:
- https://github.com/ModeShape/modeshape/blob/2.x/modeshape-jcr/src/test/resources/initialWorkspaceContent.xml - sample file with initial content
- https://github.com/ModeShape/modeshape/blob/2.x/modeshape-jcr/src/test/resources/config/configRepositoryWithInitialContent.xml - repository configuration file
these are 2 sample files from our tests.
Basically the initial content file should have it's format based on the registered node types (cnd files) - either the default node types or any custom node type, as in the above example. The the initialContent parameter values should be a path accessible to the classloader that loaded the config file.