Distribute signed integers with a bias in Matlab

Here is a sample code that distributes randomly values of -1,+1 in an array of size N

N=100;    
D = sign(rand(1,N)-0.5).*ones(1,N);

How can I control the level of bias in the distribution of the numbers?

let's say for example, that only 20% of the array will have +1, and the rest -1