Issue #3515871 by jwilson3: Fix license discrepancies from upstream rivet-core

Closes #3515871

README.md

@@ -56,3 +56,11 @@ Use the following command:
 [//]: # Do not add maintainers to cspell-project-words file
 
 * James Wilson - [jwilson3](https://www.drupal.org/u/jwilson3)
+
+## Credits
+
+This theme uses Sass variables and mixins from the
+[Rivet Design System](https://github.com/indiana-university/rivet-source)
+(BSD-3-Clause License) to compile Rivet-compatible custom CSS components, but
+does not redistribute Rivet code. Rivet's core CSS and JS assets are pulled
+directly from CDN into all rendered website pages.