adds local viewing usage to readme

README.md

-# The Official iDiv git Seminar
+# The Official iDiv git Seminars
 
 ## Usage
 
-- update reveal.js submodule
-- run make to create svg from dot sources
+To view the presentations locally, you need to clone the repository
+recursively, to also get a copy of [reveal.js][]:
+
+```bash
+git clone --recursive https://git.idiv.de/sc/edu/git-seminar.git
+```
+
+Then you also need to create the images from the [graphviz dot
+sources][graphviz]:
+
+```bash
+cd git-seminar
+make
+```
+
+Then you can view the presentations, e.g. the basics:
+
+```
+firefox basics.html
+```
 
 ## Attribution
 
 - git logo by [Jason Long](https://twitter.com/jasonlong)
+
+
+[graphviz]: http://www.graphviz.org/
+[reveal.js]: https://revealjs.com/#/