add examples/usbserial

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@990 42af7a65-404d-4744-a932-0658087f49c3

examples/README.txt

@@ -123,3 +123,9 @@ examples/pipe
  * CONFIG_EXAMPLES_PIPE_STACKSIZE
      Sets the size of the stack to use when creating the child tasks.
      The default size is 1024.
+
+examples/usbserial
+^^^^^^^^^^^^^^^^^^
+
+  This is another implementation of "Hello, World" but this one uses
+  a USB serial driver.