[slides] add readme

slides/README.md

+# DuMuX course slides
+
+The slides are automatically built and deployed in the CI and can be viewed at
+[pages.iws.uni-stuttgart.de/dumux-repositories/dumux-course/](https://pages.iws.uni-stuttgart.de/dumux-repositories/dumux-course/index.html).
+In order to add new slides, simply add your `.md` file in this folder, and add
+an entry to `index.md` such that your slides are linked from the landing page.
+__Important__: make sure to put your `.md` at the right position in the list.