Update index.md

Author: ocots

docs/src/index.md

@@ -2,11 +2,11 @@
 
 The `CTModels.jl` package is part of the [control-toolbox ecosystem](https://github.com/control-toolbox).
 
-The root package is [OptimalControl.jl](https://github.com/control-toolbox/OptimalControl.jl) which aims to provide tools to model and solve optimal control problems with ordinary differential equations by direct and indirect methods.
+The root package is [OptimalControl.jl](https://github.com/control-toolbox/OptimalControl.jl) which aims to provide tools to model and solve optimal control problems with ordinary differential equations by direct and indirect methods, both on CPU and GPU.
 
 **API Documentation**
 
 ```@contents
 Pages = Main.API_PAGES
 Depth = 1
-```
+```