doc: Note the importance of Running Status being disabled for USB

doc/usb-midi.md

@@ -0,0 +1,16 @@
+# Using the library for MIDI through USB
+
+Some boards have "native" USB functionality, which means they can directly
+connect to a computer and appear as a USB MIDI device.
+
+Here are the officially supported boards:
+
+-   Arduino Leonardo
+-   Arduino Due
+
+## Pre-requisites
+
+To use USB MIDI to send messages to the computer, you must disable Running
+Status in the settings. This is because the USB transport is stateless and has
+no knowledge of the last message's running status.
+The setting is off by default, so if you use custom settings, keep that in mind.

src/midi_Settings.h

@@ -50,6 +50,7 @@ struct DefaultSettings
 {
     /*! Running status enables short messages when sending multiple values
     of the same type and channel.\n
+    Must be disabled to send USB MIDI messages to a computer
     Warning: does not work with some hardware, enable with caution.
     */
     static const bool UseRunningStatus = false;