Added dropna to docstring for HDFStore.put()
HDFStore.put() method accepts dropna boolean parameter as it is passed
directly to the internal _write_to_group() method, but it was not
explicitly documented (as it is for append).
Just added 2 lines to the put() method docstring to mention it explicitly.  

I wondered about that, since the append() method looks to have logic specifically looking at that option, but the put() method does not (- I've not actually tested it nor dug into the methods it calls to see if it was picked up there)
 no they go the same place, guess this wasn't documented :)
