new SerialInputOutputManager.start() method

Previously recommended start action `Executors.newSingleThreadExecutor().submit(ioManager)` did not shutdown the Executor, which caused a thread leak. It's still possible to use old style start, as SerialInputOutputManager continues to implement Runnable interface.

README.md

@@ -81,7 +81,7 @@ then use direct read/write
 or direct write + event driven read:
 ```java
     usbIoManager = new SerialInputOutputManager(usbSerialPort, this);
-    Executors.newSingleThreadExecutor().submit(usbIoManager);
+    usbIoManager.start();
     ...
     port.write("hello".getBytes(), WRITE_WAIT_MILLIS);