Add Subzero as submodule.

Change-Id: I7eb124ce596d2c7e2419a01a995ea6fa871c8a88
Reviewed-on: https://swiftshader-review.googlesource.com/7290
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>
diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 0000000..be6b64f
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,3 @@
+[submodule "third_party/pnacl-subzero"]
+	path = third_party/pnacl-subzero
+	url = https://chromium.googlesource.com/native_client/pnacl-subzero