Add guides in German to the list of tutorials

[GeorgKindermann]

Maybe add two tutorials in German to the list of tutorials (learning/tutorials.md)

Grundlagen der Programmiersprache Julia: http://dx.doi.org/10.13140/RG.2.2.26626.86721

Statistik mit Julia: http://dx.doi.org/10.13140/RG.2.2.19915.98086

Grundlagen der Programmiersprache Julia and Statistik mit Julia by Georg Kindermann. Two guides in German to get started on Julia and Statistics with Julia.

Thanks!