Update README.md

README.md

@@ -21,6 +21,7 @@ Run following commands in the library directory (e.g. `extern/`) to add the libr
 git submodule add https://lab.it.hs-hannover.de/3nw-kux-u1/scg3_cmake scg3
 git submodule init
 git submodule update
+git submodule update --init --recursive
 ```