put all README.md files under CC BY-SA 4.0 License

This will make is easier for others to share README.md
files while requiring them to attribute the file
when using it.

2006/sloane/README.md

@@ -94,3 +94,11 @@ In increasing order of difficulty:
  - How is the "IOCCC 2006" logo encoded?  Can you put something else in its
    place?  Can you find a smaller encoding?
 
+## Copyright and CC BY-SA 4.0 License:
+
+This file is Copyright (c) 2023 by Landon Curt Noll.  All Rights Reserved.
+You are free to share and adapt this file under the terms this license:
+
+    Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)
+
+For more information, see: https://creativecommons.org/licenses/by-sa/4.0/