Added GMD paperwq

Author: ledm

CITATION

@@ -1 +1,12 @@
-FIXME: describe how to cite this lesson.
+---
+layout: page
+title: "Citation"
+root: .
+---
+## Citation
+
+ESMValTool can be cited as:
+
+Righi, M., Andela, B., Eyring, V., Lauer, A., Predoi, V., Schlund, M., Vegas-Regidor, J., Bock, L., Brötz, B., de Mora, L., Diblen, F., Dreyer, L., Drost, N., Earnshaw, P., Hassler, B., Koldunov, N., Little, B., Loosveldt Tomas, S., and Zimmermann, K.: ESMValTool v2.0 – Technical overview, Geosci. Model Dev. Discuss., https://doi.org/10.5194/gmd-2019-226, in review, 2019.
+
+[Link](https://www.geosci-model-dev-discuss.net/gmd-2019-226/)