Add .gitmodules file
diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 0000000..347fe93
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,3 @@
+[submodule "multibuild"]
+	path = multibuild
+	url = https://github.com/matthew-brett/multibuild.git