Give hint on how to disable the default repository.

Discussed with:	bapt
MFC after:	instantly (preparing EN)

etc/pkg/FreeBSD.conf

@@ -1,4 +1,12 @@
 # $FreeBSD$
+#
+# To disable this repository, instead of modifying or removing this file,
+# create a /usr/local/etc/pkg/repos/FreeBSD.conf file:
+#
+#   mkdir -p /usr/local/etc/pkg/repos
+#   echo "FreeBSD: { enabled: no }" > /usr/local/etc/pkg/repos/FreeBSD.conf
+#
+
 FreeBSD: {
   url: "pkg+http://pkg.FreeBSD.org/${ABI}/latest",
   mirror_type: "srv",