added entry for git submodule usage

docs/git.md

@@ -0,0 +1,19 @@
+# summary on some steps in the git workflow
+
+## Table of contents
+
+- [git submodule](#git-submodule)
+
+
+## git submodule
+```bash
+# do it manually
+git submodule init
+git submodule update
+
+# do it all at once
+git clone --recurse-submodules
+
+# if repo is already cloned
+git submodule update --init --recursive
+```