update systemd article to reference dropin file

Signed-off-by: Jessica Frazelle <acidburn@docker.com>

docs/articles/systemd.md

@@ -33,17 +33,33 @@ If you want Docker to start at boot, you should also:
 There are a number of ways to configure the daemon flags and environment variables
 for your Docker daemon.
 
-If the `docker.service` file is set to use an `EnvironmentFile`
-(often pointing to `/etc/sysconfig/docker`) then you can modify the
-referenced file.
+The recommended way is to use a systemd drop-in file. These are local files in
+the `/etc/systemd/system/docker.service.d` directory. This could also be
+`/etc/systemd/system/docker.service`, which also works for overriding the
+defaults from `/lib/systemd/system/docker.service`.
 
-Check if the `docker.service` uses an `EnvironmentFile`:
+However, if you had previously used a package which had an `EnvironmentFile`
+(often pointing to `/etc/sysconfig/docker`) then for backwards compatibility,
+you drop a file in the `/etc/systemd/system/docker.service.d`
+directory including the following:
+
+    [Service]
+    EnvironmentFile=-/etc/sysconfig/docker
+    EnvironmentFile=-/etc/sysconfig/docker-storage
+    EnvironmentFile=-/etc/sysconfig/docker-network
+    ExecStart=
+    ExecStart=/usr/bin/docker -d -H fd:// $OPTIONS \
+              $DOCKER_STORAGE_OPTIONS \
+              $DOCKER_NETWORK_OPTIONS \
+              $BLOCK_REGISTRY \
+              $INSECURE_REGISTRY
+
+To check if the `docker.service` uses an `EnvironmentFile`:
 
     $ sudo systemctl show docker | grep EnvironmentFile
     EnvironmentFile=-/etc/sysconfig/docker (ignore_errors=yes)
 
-Alternatively, find out where the service file is located, and look for the
-property:
+Alternatively, find out where the service file is located:
 
     $ sudo systemctl status docker | grep Loaded
        Loaded: loaded (/usr/lib/systemd/system/docker.service; enabled)
@@ -69,18 +85,20 @@ In this example, we'll assume that your `docker.service` file looks something li
 
     [Service]
     Type=notify
-    EnvironmentFile=-/etc/sysconfig/docker
-    ExecStart=/usr/bin/docker daemon -H fd:// $OPTIONS
+    ExecStart=/usr/bin/docker daemon -H fd://
     LimitNOFILE=1048576
     LimitNPROC=1048576
 
     [Install]
     Also=docker.socket
 
-This will allow us to add extra flags to the `/etc/sysconfig/docker` file by
-setting `OPTIONS`:
+This will allow us to add extra flags via a drop-in file (mentioned above) by
+placing a file containing the following in the `/etc/systemd/system/docker.service.d`
+directory:
 
-    OPTIONS="--graph /mnt/docker-data --storage-driver btrfs"
+    [Service]
+    ExecStart=
+    ExecStart=/usr/bin/docker daemon -H fd:// --graph /mnt/docker-data --storage-driver btrfs
 
 You can also set other environment variables in this file, for example, the
 `HTTP_PROXY` environment variables described below.