Don't raise on Vorbis error

[vindarel]

warn was used in voribis-stream.lisp:

            (unless (= output-rate rate)
              #+NIL
              (raise-vorbis-error "Open Ogg Vorbis file"
                                  "Sample rate doesn't match requested rate.")
              (warn "Sample rate doesn't match requested rate: ~:D vs expected ~:D"
                    rate output-rate))

I can scan my library, but I won't find the problematic file(s) and I don't see the warning messages…

I added a readme ;)

edit: a readme also helps for Quickdocs and such: http://quickdocs.org/mixalot/