Update README.md

Author: jackbrookes

README.md

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 # UXF - Unity Experiment Framework
 A set of components which simplify human behaviour experiments developed in the Unity engine. This is the development project, if you want to download the package, see [Releases](https://github.com/jackbrookes/unity-experiment-framework/releases/latest). 
 
+Read the [paper](https://doi.org/10.1101/459339) (preprint)
+
 If you have developed a project using UXF please [let me know](http://twitter.com/jackbrookes)! 
 
 **Contents**