Merge pull request #4710 from mitchellh/amazon-chroot-systemd

website/docs: add note about systemd and amazon-chroot.

website/source/docs/builders/amazon-chroot.html.md

@@ -116,7 +116,8 @@ each category, the available configuration keys are alphabetized.
 -   `copy_files` (array of strings) - Paths to files on the running EC2 instance
     that will be copied into the chroot environment prior to provisioning. Defaults
     to `/etc/resolv.conf` so that DNS lookups work. Pass an empty list to skip
-    copying `/etc/resolv.conf`.
+    copying `/etc/resolv.conf`. You may need to do this if you're building
+    an image that used systemd.
 
 -   `device_path` (string) - The path to the device where the root volume of the
     source AMI will be attached. This defaults to "" (empty string), which