Link to custom-elements-everywhere.com (#2039)

The site <https://custom-elements-everywhere.com> runs a suite of tests against a framework to
identify interoperability issues, and highlight potential fixes already implemented in other frameworks.

This is a great way to give more confidence while choosing Vue.

src/v2/guide/index.md

@@ -393,6 +393,8 @@ You may have noticed that Vue components are very similar to **Custom Elements**
 
 2. Vue components provide important features that are not available in plain custom elements, most notably cross-component data flow, custom event communication and build tool integrations.
 
+Although Vue doesn't use custom elements internally, it has [great interoperability](https://custom-elements-everywhere.com/#vue) when it comes to consuming or distributing as custom elements. Vue CLI also supports building Vue components that register themselves as native custom elements.
+
 ## Ready for More?
 
 We've briefly introduced the most basic features of Vue.js core - the rest of this guide will cover them and other advanced features with much finer details, so make sure to read through it all!