add @tianon as maintainer of .dockerignore

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

MAINTAINERS

@@ -6,3 +6,4 @@ Michael Crosby <michael@crosbymichael.com> (@crosbymichael)
 AUTHORS: Tianon Gravi <admwiggin@gmail.com> (@tianon)
 Dockerfile: Tianon Gravi <admwiggin@gmail.com> (@tianon)
 Makefile: Tianon Gravi <admwiggin@gmail.com> (@tianon)
+.dockerignore: Tianon Gravi <admwiggin@gmail.com> (@tianon)