Add DOI badge for system tests paper

Added a DOI badge for the publication on system tests.

Author: MakisH

tools/tests/README.md

@@ -10,6 +10,8 @@ The tutorials repository hosts cases that need multiple components from the preC
 
 Read more about the system tests in the publication [System Regression Tests for the preCICE Coupling Ecosystem](https://doi.org/10.14279/eceasst.v83.2614).
 
+[![ECEASST](https://img.shields.io/badge/DOI-10.14279%2Feceasst.v83.2614-green)](https://doi.org/10.14279/eceasst.v83.2614)
+
 ## Running the system tests
 
 The main workflow for the user is executing the `systemtests.py` script. Depending on the options given to the script, it reads in the respective metadata files and generates `docker-compose.yaml` files that can start a fully-defined coupled simulation.