Use two constraints in example problem (#688)

* Use vararg problem constructor in README.md example

* add second example with vector of constraints

* Update README.md

README.md

@@ -49,7 +49,7 @@ x = Variable(n)
 
 # The problem is to minimize ||Ax - b||^2 subject to x >= 0
 # This can be done by: minimize(objective, constraints)
-problem = minimize(sumsquares(A * x - b), [x >= 0])
+problem = minimize(sumsquares(A * x - b), [x >= 0, x <= 1])
 
 # Solve the problem by calling solve!
 solve!(problem, SCS.Optimizer)