Update README.rst

[lenguyenthedat]

Need only_unknowns=True when using svd.recommend() to recommend only non-rated movies to a user

[ocelma]

If you change the 'only_unknowns' parameter from False to True in the example, then you also need to update the results (the movies recommended).
They'll be different as it is now (some will be filtered out).

[lenguyenthedat]

I see, will change this accordingly.

On another note, it seems like we can't use only_unknowns=True when loading svd by the SVDLIBC method (i.e step#8 at https://github.com/ocelma/python-recsys/blob/master/doc/source/quickstart.rst)

Do you have any workaround for it?