simplification of sums with binomial coefficients



Hello,
How do I make Maxima simplify
sum(r/k*binom(n,r)*binom(m,k-r)/binom(n+m,k),r,0,k) ?

simpsum doesn't seem to help. Maybe somebody can give an example of the 
Zeilberger
share package usage in this situation?

--
Andrei Zorine