Merge branch 'feature/slides' into 'master'

[slides][2018] add slides for exercise main, runtime, grids

See merge request !38

slides/README.md

 # DuMu<sup>x</sup> course slides
+
+Click on the links to download the slides
+
+* [Slides on the basics]()
+* [Slides on the main file](./dumux-course-testproblem.pdf)
+* [Slides on runtime parameters and grids](./dumux-course-runtimeparams-grids.pdf)