README: add facebook follow button
Facebook does not have a public API for that, so we have to do it by-hand...
see:
@@ -212,7 +212,9 @@ For a more detailed technical documentation you can head over to our [godocs](ht
<a href="https://iris-go.com/#book"> <img alt="Book cover" src="https://iris-go.com/images/iris-book-cover-sm.jpg?v=12" /> </a>
-[![follow Iris web framework on twitter](https://img.shields.io/twitter/follow/iris_framework?color=ee7506&logoColor=ee7506&style=for-the-badge)](https://twitter.com/intent/follow?screen_name=iris_framework)
+[![follow Iris web framework on twitter](https://img.shields.io/twitter/follow/iris_framework?color=ee7506&logoColor=ee7506&style=for-the-badge&logo=twitter)](https://twitter.com/intent/follow?screen_name=iris_framework)
+
+[![follow Iris web framework on facebook](https://img.shields.io/badge/Follow%20%40Iris.framework-445-2D88FF.svg?style=for-the-badge&logo=facebook)](https://www.facebook.com/iris.framework)
You can [request](https://iris-go.com/#book) a PDF version and online access of the **E-Book** today and be participated in the development of Iris.
