apps/gophers,apps/hello: add references to gioui.org

Signed-off-by: Elias Naur <mail@eliasnaur.com>

apps/gophers/main.go

@@ -2,6 +2,8 @@
 
 package main
 
+// A Gio program that displays Go contributors from GitHub. See https://gioui.org for more information.
+
 import (
 	"context"
 	"flag"

apps/hello/hello.go

@@ -2,6 +2,8 @@
 
 package main
 
+// A simple Gio program. See https://gioui.org for more information.
+
 import (
 	"image"
 	"image/color"