Extend references

docs/src/references.md

@@ -7,6 +7,8 @@ with good documentation for explicit and implicit problems.
 ## Julia
 - [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 open-source and commercial 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) is 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. 
+- [KiteModels.jl](https://ufechner7.github.io/KiteModels.jl/stable/) implements kite models, connected to a tether for airborne wind energy applications. It uses the same algorithms as this tutorial, but it is not (yet) using ModelingToolkit. 
 
 ## 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.
+- Uwe Fechner, Rolf van der Vlugt, Edwin Schreuder, Roland Schmehl. (2015). [*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.
+- Yingbo Ma, Shashi Gowda, Ranjan Anantharaman, Chris Laughman, Viral Shah, and Chris Rackauckas. (2021). [*ModelingToolkit: A Composable Graph Transformation System For Equation-Based Modeling.*](https://arxiv.org/abs/2103.05244)