Welcome to Stackoverflow,
like the exception already tells you, the file you want to be committed is not under version control yet, so you have to add it first.
On your CommitEditor
you should call something like this:
SVNRepository repository = SVNRepositoryFactory.create( SVNURL.parseURIDecoded( url ) );
ISVNEditor editor = repository.getCommitEditor( logMessage , null /*locks*/ , true /*keepLocks*/ , null /*mediator*/ );
//the second and the third parameters are the path and revision respectively
//of the item's ancestor if the item is being added with history
editor.addFile( "your/path/project/README.txt" , null , -1 );
You should also have a look at the full example at SVNKit