Add Michael Grund to AUTHORS.md
It's great to have the opportunity contributing to this project since I used GMT for years. I hope to contribute more in the future  ;)
@@ -8,6 +8,7 @@ Manoa.
The following people have contributed code to the project (alphabetical by last name)
and are considered the "PyGMT Developers":
+* [Michael Grund](https://github.com/michaelgrund)
* [Wei Ji Leong](https://github.com/weiji14)
* [Tyler Newton](http://www.tnewton.com/)
* [William Schlitzer](https://github.com/willschlitzer)
