Add notebook showing how to process data with missing vals.

README.md

@@ -20,3 +20,4 @@ To be able to run all notebooks install the kernel specs in your home on mistral
 ## List of Content:
 
 * [Masking Regions in gridded data sets using shapefiles](https://gitlab.dkrz.de/freva/codesnippets/-/blob/master/notebooks/MaskRegionFromShapeFile.ipynb)
+* [Reduce the time dimension (mean or sum) of data with two types of different missing values](https://gitlab.dkrz.de/freva/plugins4freva/codesnippets/-/blob/master/notebooks/AvgOrSumDataWithMissingValues.ipynb)