Add comment regarding source of code for reproducible zip archiving i…

…n `zip_bag_dir`. [ci skip]

bdbag/bdbag_api.py

@@ -420,7 +420,8 @@ def filter_mtime(tarinfo):
 
 
 def zip_bag_dir(bag_path, zip_file_path, idempotent=False):
-
+    # The majority of this code came from https://fekir.info/post/reproducible-zip-archives/ with the exception of the
+    # buffered writing of file entries (instead of ZipFile.writestr) which was added for scalability reasons.
     zipfile = ZipFile(zip_file_path, 'w', ZIP_DEFLATED, allowZip64=True)
     entries = []
     for root, dirs, files in os.walk(bag_path):