Add requirements to nva_1970.json (#362)

A note that this faction requires WW2 Assets Pack because of ze Schnellboot

resources/factions/nva_1970.json

@@ -50,6 +50,9 @@
     "ZU-23 on Ural-375",
     "ZSU-23-4 Shilka"
   ],
+  "requirements": {
+    "WW2 Asset Pack": "https://www.digitalcombatsimulator.com/en/products/other/wwii_assets_pack/"
+  },
   "requirements": {},
   "doctrine": "coldwar"
-}
+}