Added a prelude

readme.md

@@ -34,10 +34,7 @@ For now, here is a simple example of using the propagator to solve a simple syst
 
 ```rust
 use nalgebra::Vector3;
-use differential_equations::integrator::dormand_prince::DormandPrince45;
-use differential_equations::controller::PIController;
-use differential_equations::callback::stop;
-use differential_equations::problem::*;
+use differential_equations::prelude::*;
 
 // Define the system (parameters, derivative, and initial state)
 type Params = (f64, bool);