Add missing link to ReadMe

README.md

@@ -10,6 +10,8 @@ The main task of the exercise is to introduce model transformations **for the do
 
 In this exercise, we will focus on model-to-model transformations using the *Epsilon* language family[^epsilon], particularly the **Epsilon Transformation Language (ETL)**, to transform, rewrite, and extend our *transportation network* models.
 
+[^epsilon]: [https://www.eclipse.org/epsilon/](https://www.eclipse.org/epsilon/)
+
 1. At first, you need to prepare an Epsilon ETL environment:
 
     1. Install `Epsilon 2.4` from the `Eclipse Marketplace`