Added example for 'mix' method with weight param

I was going to keep using some my old method (functionality of which I did not find in your lib). But after digging in source code for another reason I noticed that 'mix' method has 2 params, and 2nd is optional. After playing a bit with it I realized that 'mix' method does exactly same as my method, but in a bit different manner.

I think would be nice to let user know about 2nd param without digging into sources