Update README.md

Author: malteschaefer1

README.md

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 An in-depth study into the methodological aspects of grid emission factor calculations. This code is the computational engine under the hood of the study "Towards Standardized Grid Emission Factors: Methodological Insights and Best Practices" to be submitted soon. The preprint can be found at [arXiv](https://arxiv.org/abs/2311.01103). Find a release version of the code [here](https://doi.org/10.24355/dbbs.084-202309131139-0) and of the data [here](https://doi.org/10.24355/dbbs.084-202309111514-0).
 
+![ef_histogram](https://github.com/malteschaefer1/EFmethodmadness/assets/29174285/9ce58eb3-d855-412e-82fc-19336649403c)
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 ## Table of Contents
 - [Installation](#installation)
 - [Usage](#usage)