docs: explain how to configure the Vue version (#448)

Co-authored-by: Anthony Fu <anthonyfu117@hotmail.com>

README.md

@@ -385,6 +385,36 @@ export default antfu()
 // ...
 ```
 
+### Vue
+
+Vue support is detected automatically by checking if `vue` is installed in your project. You can also explicitly enable/disable it:
+
+```js
+// eslint.config.js
+import antfu from '@antfu/eslint-config'
+
+export default antfu({
+  vue: true
+})
+```
+
+#### Vue 2
+
+We have limited support for Vue 2 (as it's already [reached EOL](https://v2.vuejs.org/eol/)). If you are still using Vue 2, you can configure it manually by setting `vueVersion` to `2`:
+
+```js
+// eslint.config.js
+import antfu from '@antfu/eslint-config'
+
+export default antfu({
+  vue: {
+    vueVersion: 2
+  },
+})
+```
+
+As it's in maintenance mode, we only accept bug fixes for Vue 2. It might also be removed in the future when `eslint-plugin-vue` drops support for Vue 2. We recommend upgrading to Vue 3 if possible.
+
 ### Optional Configs
 
 We provide some optional configs for specific use cases, that we don't include their dependencies by default.