Readme: move gitter-chat to a more representive place.

Readme.md

@@ -15,6 +15,7 @@ Coverity<a href="https://scan.coverity.com/projects/5607">
        src="https://scan.coverity.com/projects/5607/badge.svg"/>
 </a>
 <br>
+Gitter chat: [![Join the chat at https://gitter.im/assimp/assimp](https://badges.gitter.im/assimp/assimp.svg)](https://gitter.im/assimp/assimp?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)<br>
 __[open3mod](https://github.com/acgessler/open3mod) is a powerful 3D model viewer based on Assimp's import and export abilities.__
 
 #### Supported file formats ####