Add current contributors to the AUTHORS file (fatiando#141)

As instructed by the new authorship guidelines (fatiando/community#13)
contributors who wish to be included in Zenodo archives need to be listed
in `AUTHORS.md` and include affiliation and ORCID. Include current
contributors in the file so it matches what we will put in Zenodo.

Author: santisoler

AUTHORS.md

@@ -1,7 +1,11 @@
 # Project Authors
 
-The following people have made contributions to the project (in alphabetical order by
-last name) and are considered "The Harmonica Developers":
+The following people have made contributions to the project (in alphabetical
+order by last name) and are considered "The Harmonica Developers":
 
-* [Santiago Soler](https://github.com/santisoler)
-* [Leonardo Uieda](https://github.com/leouieda)
+* [Vanderlei C Oliveira Jr](https://github.com/birocoles) - Observatório Nacional, Brazil (ORCID: 0000-0002-6338-4086)
+* [Agustina Pesce](https://github.com/aguspesce) - Universidad Nacional de San Juan, Argentina (ORCID: 0000-0002-5538-8845)
+* [Nicholas Shea](https://github.com/nshea3) - MINES ParisTech (ORCID: 0000-0001-6007-5686)
+* [Santiago Soler](https://github.com/santisoler) - CONICET, Argentina; Instituto Geofísico Sismológico Volponi, Universidad Nacional de San Juan, Argentina (ORCID: 0000-0001-9202-5317)
+* [Leonardo Uieda](https://github.com/leouieda) - University of Liverpool, UK (ORCID: 0000-0001-6123-9515)
+* [ziebam](https://github.com/ziebam) (not included in Zenodo)