Add circuit off main disconnect or sub-panel required?
You may feed a 14-50 outlet directly from your main panel since there are unused breaker slots available.  Use the correct breaker of course, and correct conductor type/size for the load.  Do not forget voltage drop and temperature ratings when choosing cable.  

You do not need to add another sub-panel for a single outlet.

Do not expect the code book to tell you what you can do.  It's there to tell you what you can't.