Update references.

docs/src/references.md

@@ -5,7 +5,8 @@
 with good documentation for explicit and implicit problems.
 
 ## Julia
-[DifferentialEquations](https://docs.sciml.ai/DiffEqDocs/stable/) offers a unified interface to about 150 different solvers for a large range of problems. It wraps many existing solvers, that have been implemented in C++ or Fortran and adds a growing number of native Julia solvers, many of them state-of-the-art.
+- [DifferentialEquations.jl](https://docs.sciml.ai/DiffEqDocs/stable/) offers a unified interface to about 150 different solvers for a large range of problems. It wraps many existing solvers, that have been implemented in C++ or Fortran and adds a growing number of native Julia solvers, many of them state-of-the-art.
+- [ModelingToolkit.jl](https://github.com/SciML/ModelingToolkit.jl)An acausal modeling framework for automatically parallelized scientific machine learning (SciML) in Julia. A computer algebra system for integrated symbolics for physics-informed machine learning and automated transformations of differential equations. 
 
 ## Scientific papers
 - The paper [Dynamic Model of a Pumping Kite Power System](http://arxiv.org/abs/1406.6218) describes the tether model used in this tutorial, but also a model of a complete kite power system with experimental validation.