Add author

- The guidelines repository require to add your name as contributor in
the AUTHOR file
- This commit adds the new contributor

Author: Franck Rupin

AUTHORS

@@ -13,3 +13,4 @@ Tejas Kokje <tkokje@digitalocean.com>
 Kei Nohguchi <knohguchi@digitalocean.com>
 Neal Shrader <nshrader@digitalocean.com>
 Sangeetha Srikanth <ssrikanth@digitalocean.com>
+Franck Rupin <frupin@digitalocean.com>