add citations

Author: SarahAlidoost

CITATION

@@ -1 +1,15 @@
-FIXME: describe how to cite this lesson.
+Please cite this tutorial as:
+
+- ESMValTool 2.0 tutorial, ESMValTool developers, version [ADD VERSION],
+  [https://github.com/ESMValGroup/ESMValTool_tutorial](https://github.com/ESMValGroup/ESMValTool_tutorial),
+  [DATE].
+
+and cite ESMValTool as:
+
+- Lauer, A., Eyring, V., Bellprat, O., Bock, L., Gier, B. K., Hunter, A.,
+  Lorenz, R., Pérez-Zanón, N., Righi, M., Schlund, M., Senftleben, D., Weigel,
+  K., and Zechlau, S.: Earth System Model Evaluation Tool (ESMValTool) v2.0 –
+  diagnostics for emergent constraints and future projections from Earth
+  system models in CMIP, Geosci. Model Dev. Discuss.,
+  [https://doi.org/10.5194/gmd-2020-60](https://doi.org/10.5194/gmd-2020-60),
+  in review, 2020.