docs: note that lightningcss minification needs the optional peer

Author: danielroe

docs/content/1.get-started/5.upgrade.md

@@ -53,7 +53,13 @@ Font metadata and downloaded font files are now cached in `node_modules/.cache/n
 
 #### Injected `@font-face` rules are minified with lightningcss
 
-Generated `@font-face` declarations were previously minified with esbuild unless you had opted into `css.lightningcss`. They are now always minified with lightningcss, so the exact serialisation of the CSS we inject may differ (for example `local(Font Name)` rather than `local("Font Name")`). This is cosmetic, but it will show up in snapshot tests.
+Generated `@font-face` declarations were previously minified with esbuild unless you had opted into `css.lightningcss`. They are now minified with lightningcss, so the exact serialisation of the CSS we inject may differ (for example `local(Font Name)` rather than `local("Font Name")`). This is cosmetic, but it will show up in snapshot tests.
+
+lightningcss is an optional peer dependency, as its native binary is several megabytes. Nuxt v4.5 (with Vite 8) brings this in, but if you are using an earlier version of Nuxt, you will need to install it manually to enable css minification.
+
+```bash [Terminal]
+npm install --save-dev lightningcss
+```
 
 #### Font failures fail production builds