Update README.md

Author: jbnsn

README.md

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 The file [`kitzes-2013.py`](kitzes-2013.py) contains a most simple implementation of [Kitzes (2013)](https://doi.org/10.3390/resources2040489) in Python with the packages [NumPy](https://numpy.org/) and [Pandas](https://pandas.pydata.org/). It supplements [Bunsen and Finkbeiner (2013)](https://doi.org/10.3390/su15010046) and is also available via Zenodo ([Bunsen 2022](https://doi.org/10.5281/zenodo.7431089)). When working with this script, a reference is not mandatory.
 
+## References
 > Bunsen, Jonas. 2022. ‘A Most Simple Implementation of Kitzes (2013) in Python’. Zenodo. https://doi.org/10.5281/zenodo.7431089.
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 > Bunsen, Jonas, and Matthias Finkbeiner. 2023. ‘An Introductory Review of Input-Output Analysis in Sustainability Sciences Including Potential Implications of Aggregation’. Sustainability 15 (1): 46. https://doi.org/10.3390/su15010046.
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 > Kitzes, Justin. 2013. ‘An Introduction to Environmentally-Extended Input-Output Analysis’. Resources 2 (4): 489–503. [https://doi.org/10.3390/resources2040489](https://doi.org/10.3390/resources2040489).
 
 _Note: [Pymrio](https://github.com/konstantinstadler/pymrio) is a comprehensive package for Multi-Regional Input-Output Analysis (MRIO) in Python._