add instructions to clone with submodules

README.md

@@ -16,7 +16,7 @@ In order to use the slide deck, you must have:
 - Fork this repository
 - Clone your fork
     ```bash
-    $ git clone ssh://git@git-r3lab-server.uni.lu:8022/<yourUsername>/presentations.git
+    $ git clone --recurse-submodules ssh://git@git-r3lab-server.uni.lu:8022/<yourUsername>/presentations.git
     ```
 - Open a terminal and work on your presentation:
     ```bash