sysctl.d: Include a README file for /etc/sysctl.d/

Our sysctl script has read from /etc/sysctl.d/ but the directory was not
created by default, and we didn't document it.

X-Gentoo-Bug: 398189
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=398189
Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>

mk/sys.mk

@@ -22,6 +22,7 @@ SYSCONFDIR?=		${PREFIX}/etc
 INITDIR?=		${SYSCONFDIR}/init.d
 CONFDIR?=		${SYSCONFDIR}/conf.d
 LOCALDIR?=		${SYSCONFDIR}/local.d
+SYSCTLDIR?=		${SYSCONFDIR}/sysctl.d
 
 BINDIR?=		${PREFIX}/sbin
 BINMODE?=		0755