add submodule sync
diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 0000000..cd321e8
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,3 @@
+[submodule "sync"]
+	path = sync
+	url = git@github.com:zhenkai/sync.git