Add spdx licenses as a submodule

Author: ebroto

Diff for: .gitmodules

@@ -0,0 +1,3 @@
+[submodule "resources/licenses/spdx"]
+	path = resources/licenses/spdx
+	url = https://github.com/spdx/license-list-data

Diff for: build.rs

@@ -16,7 +16,7 @@ fn main() {
     let mut store = Store::new();
     store
         .load_spdx(Path::new("resources/licenses/spdx/json/details"), false)
-        .expect("Couldn't create a store from SPDX data");
+        .expect("Couldn't create a store from SPDX data. Have submodules been initialized?");
 
     let mut cache = File::create(EMBEDDED_CACHE).unwrap();
     store.to_cache(&mut cache).unwrap();