Update instructions about NSIS packaging

Include a helper a script to gather valid Qt translations for packaging.

dist/windows/gather_qt_translations.py

@@ -0,0 +1,31 @@
+#!/usr/bin/env python3
+
+import argparse
+import glob
+import os
+import shutil
+import sys
+from typing import List
+
+def isNotStub(path: str) -> bool:
+    return (os.path.getsize(path) >= (10 * 1024))
+
+def main() -> int:
+    parser = argparse.ArgumentParser(description='Gather valid Qt translations for NSIS packaging.')
+    parser.add_argument("qt_translations_folder", help="Qt's translations folder")
+    parser.add_argument("nsis_packaging_folder", help="NSIS packaging translations folder")
+    args = parser.parse_args()
+
+    tmp_translations: List[str] = glob.glob(f'{args.qt_translations_folder}/qt_??.qm')
+    tmp_translations += glob.glob(f'{args.qt_translations_folder}/qt_??_??.qm')
+    tmp_translations += glob.glob(f'{args.qt_translations_folder}/qtbase_??.qm')
+    tmp_translations += glob.glob(f'{args.qt_translations_folder}qtbase_??_??.qm')
+
+    filtered = filter(isNotStub, tmp_translations)
+    for file in filtered:
+        shutil.copy2(file, args.nsis_packaging_folder)
+
+    return 0
+
+if __name__ == '__main__':
+    sys.exit(main())