documentation for license notes

docs/CONTRIBUTING.md

@@ -140,6 +140,22 @@ Example:
     ```text
     GOOD: * [A Way-backed Interesting Book](https://web.archive.org/web/20211016123456/http://example.com/) - John Doe (HTML) *(:card_file_box: archived)*
     ```
+- <a id="license"></a>Free Licenses (While we include resources that are "All Rights Reserved" but free to read, we encourage the use of free Licenses, such as Creative Commons):
+
+    ```text
+    GOOD: * [A Very Awesome Book](https://example.org/book.pdf) - Jane Roe (PDF) (CC BY-SA)
+    ```
+
+Supported Licences (no versioning):
+
+    - `CC BY` 'Creative Commons attribution'
+    - `CC BY-NC` 'Creative Commons non-commercial'
+    - `CC BY-SA` 'Creative Commons share-alike'
+    - `CC BY-NC-SA` 'Creative Commons non-commercial, share-alike'
+    - `CC BY-ND` 'Creative Commons no-derivatives'
+    - `CC BY-NC-ND` 'Creative Commons non-commercial, no-derivatives'
+    - `GFDL` 'Gnu Free Documentation License'
+
 
 ### Alphabetical order