README and presolve

[trulsf]

In the README file it it is stated: To use Xpress with JuMP, use:

using JuMP, Xpress
model = Model(Xpress.Optimizer)
set_optimizer(model, "PRESOLVE", 0)

I guess, it should be set_attribute(model, "PRESOLVE", 0), but the question is: Is it recommended to turn off presolve as a default for Xpress? What features of JuMP will require this (if any)?

[odow]

Fixed by #280.

The PRESOLVE was just an example. It's not required or recommended. I've clarified that it is just an example.