doc: use :Contact: instead of :Author: in the README

With :Author:, people may think I am the only author (which is not the case,
as shown by the Git history), whereas I just wanted to specify who is the lead
developer, i.e. the person to contact. And it's not a place to list here people
that are not involved anymore in the project.

README.rst

@@ -6,7 +6,7 @@
 Resilient, Scalable, IPv6 Network application
 ---------------------------------------------
 
-:Author: Julien Muchembled <jm@nexedi.com>
+:Contact: Julien Muchembled <jm@nexedi.com>
 
 Overview
 ========