Update README handle_cmd comments
Update README handle_cmd comments

Update REAMDE handle_cmd comments about the handle_cmd
passthrough support for handlers that also implement
read/write/flush callouts.

lxbsz/tcmu-runner,JefferyHu/tcmu-runner,liuy/tcmu-runner,agrover/tcmu-runner,open-iscsi/tcmu-runner,superlibj/tcmu-runner,open-iscsi/tcmu-runner