Docu update.

CITATION

 If you use the Climate Data Operators (CDO) to process data for an article in a scientific publication, please cite:
 
-Schulzweida, Uwe. (2019, October 31). CDO User Guide (Version 1.9.8). Zenodo. http://doi.org/10.5281/zenodo.3539275
+Schulzweida, Uwe. (2020, October 31). CDO User Guide (Version 1.9.9). Zenodo. http://doi.org/10.5281/zenodo.4246983
 
 BibTeX:
-@misc{schulzweida_uwe_2019_3539275,
+@misc{schulzweida_uwe_2020_4246983,
   author       = {Schulzweida, Uwe},
   title        = {CDO User Guide},
   month        = oct,
-  year         = 2019,
-  doi          = {10.5281/zenodo.3539275},
-  url          = {https://doi.org/10.5281/zenodo.3539275}
+  year         = 2020,
+  publisher    = {Zenodo},
+  version      = {1.9.9},
+  doi          = {10.5281/zenodo.4246983},
+  url          = {https://doi.org/10.5281/zenodo.4246983}
 }