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>

sysctl.d/README

@@ -0,0 +1,13 @@
+Kernel system variables configuration files
+
+Files found under the /etc/sysctl.d directory that end with .conf are
+parsed within sysctl(8) at boot time.  If you want to set kernel variables
+you can either edit /etc/sysctl.conf or make a new file.
+
+The filename isn't important, but don't make it a package name as it may clash
+with something the package builder needs later. It must end with .conf or else
+it will not be read.
+
+The recommended locationfor local system settings is /etc/sysctl.d/local.conf
+but as long as you follow the rules for the names of the file, anything will
+work. See sysctl.conf(5) man page for details of the format.