Add reference to the figure.

README.md

@@ -5,6 +5,7 @@
 SCOPE is a radiative transfer and energy balance model, and STEMMUS model is a two-phase mass and heat transfer model. For more information about the coupling between these two models, please check [this reference](https://gmd.copernicus.org/articles/14/1379/2021/).
 
 <img width="500" alt="Logo" src=./docs/assets/imgs/coupling_scheme.png>
+(by Zeng & Su, 2021)
 
 ## Running STEMMUS_SCOPE
 
@@ -42,4 +43,4 @@ have a look at the [contribution guidelines](CONTRIBUTING.md).
 <!-- [![DOI](https://zenodo.org/badge/DOI/<replace-with-created-DOI>.svg)](https://doi.org/<replace-with-created-DOI>) -->
 
 <!--TODO: add links to zenodo. -->
-More information will follow soon.
+More information will follow soon.