add link to old docs

Author: yyx990803

docs/en/SUMMARY.md

@@ -2,6 +2,7 @@
 
 > Note: [email protected] only works with Vue 2.x. The docs for the 1.x router is [here](https://github.com/vuejs/vue-router/tree/1.0/docs/en).
 
+- [Looking for 0.7 Docs?](https://github.com/vuejs/vue-router/tree/1.0/docs/en)
 - [Release Notes](https://github.com/vuejs/vue-router/releases)
 - [Installation](installation.md)
 - Essentials