(docs) note location of CSS has changed with v11

Author: joshgoebel

VERSION_11_UPGRADE.md

@@ -63,6 +63,7 @@ hljs.registerAliases(["php3","php4","php5","php6","php7","php8"],{ languageName:
 - `github` includes significant changes to more properly match modern GitHub syntax highlighting. If you desire the old theme you can manually copy it into your project from the [10-stable branch](https://github.com/highlightjs/highlight.js/tree/10-stable/src/styles).
 - `github-gist` has been removed in favor of `github` as GitHub and GitHub Gist have converged. If you desire the theme you can manually copy it into your project from the [10-stable branch](https://github.com/highlightjs/highlight.js/tree/10-stable/src/styles).
 - The `.hljs` CSS selector is now further scoped.  It now targets `code.hljs` (inline code) and `pre code.hljs` (code blocks). If you are using a different element you may need to update your CSS to reapply some styling.
+- All [Base16 themes](https://github.com/highlightjs/base16-highlightjs) now live in the `styles/base16` directory - this means some CSS files have moved.  Please confirm themes you use reference the new locations. 
 
 
 #### Grammar Scopes