Add the Creative Commons link to the footer.

Author: amartini51

Sources/TSPL/TSPL.docc/footer.html

@@ -13,6 +13,7 @@
   <div class="container">
     <div>
       <p class="copyright">Copyright © 2022 Apple Inc. All rights reserved.</p>
+      <p>This document is made available under a <a rel="license" href="https://creativecommons.org/licenses/by/4.0/">Creative Commons Attribution 4.0 International (CC BY 4.0) License</a>.</p>
       <p class="trademark">Swift and the Swift logo are trademarks of Apple Inc.</p>
       <p class="privacy">
         <a href="//www.apple.com/privacy/privacy-policy/">Privacy Policy</a>