Modified ubuntulinux.md for instructions to configure DNS for Ubuntu 15.04 or greater

modified note to provide a link for installation instructions on Ubuntu 15.04 or above

cleaned up typos for ubuntulinux.md

Signed-off-by: Zuhayr Elahi <elahi.zuhayr@gmail.com>

docs/installation/ubuntulinux.md

@@ -349,6 +349,11 @@ To avoid this warning, you can specify a DNS server for use by Docker
 containers. Or, you can disable `dnsmasq` in NetworkManager. Though, disabling
 `dnsmasq` might make DNS resolution slower on some networks.
 
+The instructions below describe how to configure the Docker daemon running on
+Ubuntu 14.10 or below. Ubuntu 15.04 and above use systemd as its boot
+and service manager. Refer to [control and configure Docker with systemd](../articles/systemd.md#custom-docker-daemon-options)
+to configure a daemon controled by systemd.
+
 To specify a DNS server for use by Docker:
 
 1. Log into Ubuntu as a user with `sudo` privileges.