Docs: update README.md

Author: mysticatea

README.md

@@ -6,6 +6,14 @@
 
 > Official ESLint plugin for Vue.js
 
+## :exclamation: Attention - this is documentation for version `5.x` :exclamation:
+
+This branch contains `eslint-plugin-vue@next` which is a pre-released `5.0`, but it's not the default version that you get with `npm install eslint-plugin-vue`. In order to install this you need to specify either `"eslint-plugin-vue": "next"` in `package.json` or do `npm install eslint-plugin-vue@next`.
+
+Please try it and report any issues that you might have encountered.
+
+If you want to check previous releases [go here](https://github.com/vuejs/eslint-plugin-vue/releases).
+
 ## :art: Playground on the Web
 
 You can try this plugin on the Web.
@@ -20,7 +28,7 @@ You can try this plugin on the Web.
 ## :cd: Installation
 
 ```bash
-npm install --save-dev eslint eslint-plugin-vue
+npm install --save-dev eslint eslint-plugin-vue@next
 ```
 
 ## :rocket: Usage