docs: document library external globals

docs/guide/build.md

@@ -97,7 +97,16 @@ module.exports = {
       name: 'MyLib'
     },
     rollupOptions: {
-      external: ['vue']
+      // make sure to externalize deps that shouldn't be bundled
+      // into your library
+      external: ['vue'],
+      output: {
+        // Provide global variables to use in the UMD build
+        // for externalized deps
+        globals: {
+          vue: 'Vue'
+        }
+      }
     }
   }
 }