AUTHORS: Add Martin Xu.

Signed-off-by: Ben Pfaff <blp@ovn.org>

AUTHORS.rst

@@ -237,6 +237,7 @@ Markos Chandras                    mchandras@suse.de
 Martin Casado                      casado@cs.stanford.edu
 Martin Fong                        mwfong@csl.sri.com
 Martino Fornasa                    mf@fornasa.it
+Martin Xu                          martinxu9.ovs@gmail.com
 Maryam Tahhan                      maryam.tahhan@intel.com
 Matteo Croce                       mcroce@redhat.com
 Mauricio Vásquez                   mauricio.vasquezbernal@studenti.polito.it