Clarify licensing status in README (#20)

* Update README.md

* Fix punctuation

Co-authored-by: Antonio Vivace <avivace4@gmail.com>

Co-authored-by: Antonio Vivace <avivace4@gmail.com>

Author: ISSOtm

README.md

@@ -51,4 +51,8 @@ $ cp build/highlight.min.js ../gb-asm-tutorial/theme/highlight.js
 
 ## License
 
-We assume the content to be in the [public domain](LICENSE).
+Different parts of gb-asm-tutorial are subject to different licenses:
+
+- All the code contained within the tutorial itself is licensed under <a rel="license" href="http://creativecommons.org/publicdomain/zero/1.0/">CC0</a>. *To the extent possible under law, all copyright and related or neighboring rights to code presented within GB ASM Tutorial have been waived. This work is published from France.*
+- The contents (prose, images, etc.) of this tutorial are licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-sa/4.0/">Creative Commons Attribution-ShareAlike 4.0 International License</a>.
+- Code used to display and format the site is licensed under the [MIT License](https://github.com/ISSOtm/gb-asm-tutorial/blob/master/LICENSE) unless otherwise specified.