Add _target="blank" deprecation on <router-link> (#539)

* Add _target="blank" deprecation on `<router-link>`

* Tweak language about route links in new tabs

Author: fnlctrl

src/guide/migration-vue-router.md

@@ -267,6 +267,8 @@ You'll need to update it like this:
 <router-link to="/about">About</router-link>
 ```
 
+Note that `target="_blank"` is not supported on `<router-link>`, so if you need to open a link in a new tab, you have to use `<a>` instead.
+
 {% raw %}
 <div class="upgrade-path">
   <h4>Upgrade Path</h4>