Added SDL_JoystickCurrentPowerLevel() to WhatsNew.txt for 2.0.4.

WhatsNew.txt

@@ -33,6 +33,7 @@ General:
 * Renamed SDL_assert_data to SDL_AssertData and SDL_assert_state to SDL_AssertState
 * Added a hint SDL_HINT_WINDOW_FRAME_USABLE_WHILE_CURSOR_HIDDEN to prevent window interaction while cursor is hidden
 * Added SDL_GetDisplayDPI() to get the DPI information for a display
+* Added SDL_JoystickCurrentPowerLevel() to get the battery level of a joystick
 
 Windows:
 * Added support for Windows Phone 8.1