[slides][intro] Add dumux folder structure.

slides/intro.md

@@ -100,6 +100,21 @@ We acknowledge funding that supported the development of DuMu^x^ in past and pre
 
 <img src="img/lines_vs_releases.png" width="600"/>
 
+## Folder structure
+
+```code
+dumux
+├── build-cmake
+│   ├── examples
+│   ├── test
+│   ├── ...
+├── doc
+├── dumux
+├── examples
+├── test
+└── ...
+```
+
 ## Mailing Lists and GitLab
 
 * **Mailing lists** of DUNE (<dune@dune-project.org>) and DuMu^x^ (<dumux@listserv.uni-stuttgart.de>)