[slides] update README

slides/README.md

@@ -4,4 +4,4 @@ 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.
+__Important__: In `index.md`, link to your slides using `./YOUR_FILENAME.html`, and make sure to put your link at the right position in the list.