add Tianon as maintainer of AUTHORS and .mailmap

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)

MAINTAINERS

@@ -2,6 +2,8 @@ Solomon Hykes <solomon@docker.com> (@shykes)
 Guillaume J. Charmes <guillaume@docker.com> (@creack)
 Victor Vieux <vieux@docker.com> (@vieux)
 Michael Crosby <michael@crosbymichael.com> (@crosbymichael)
+.mailmap: Tianon Gravi <admwiggin@gmail.com> (@tianon)
 .travis.yml: Tianon Gravi <admwiggin@gmail.com> (@tianon)
+AUTHORS: Tianon Gravi <admwiggin@gmail.com> (@tianon)
 Dockerfile: Tianon Gravi <admwiggin@gmail.com> (@tianon)
 Makefile: Tianon Gravi <admwiggin@gmail.com> (@tianon)