Checkout submodules with shallow depth (#1353)

* Make submodules shallow

* Updated README for the shallow checkout changes.

.gitmodules

@@ -2,7 +2,9 @@
 	path = ext-src/mscclpp
 	url = https://github.com/microsoft/mscclpp.git
 	ignore = dirty
+	shallow = true
 [submodule "ext-src/json"]
 	path = ext-src/json
 	url = https://github.com/nlohmann/json.git
 	ignore = dirty
+	shallow = true