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.
@@ -75,8 +75,8 @@ difference is who is responsible for the event loop.
There are two different ways to write a tcmu-runner plugin handler:
1. one can register .handle_cmd to take the full control of command handling
-2. or else one can register .{read, write, flush, ...} to only handle storage
- IO stuff.
+2. or else one can register .{read, write, flush, ...} to handle specific
+ operations and .handle_cmd to override the generic handling if needed.
With the option 1, tcmu-runner is in charge of the event loop
for your plugin, and your handler's `handle_cmd` function is called
