Update README to remove explicit references to chain attribute

The chain name is derived from the resource name. To specify the chain from
which this chain should be jumped to, use the `direction` attribute.

resolves #49

README.md

@@ -219,24 +219,15 @@ Suppose you had the following `simple_iptables` configuration:
     # for logical organization.
 
     # Allow all traffic on the loopback device
-    simple_iptables_rule "loopback" do
-      chain "system"
-      rule "--in-interface lo"
-      jump "ACCEPT"
-    end
-
-    # Allow any established connections to continue, even
-    # if they would be in violation of other rules.
-    simple_iptables_rule "established" do
-      chain "system"
-      rule "-m conntrack --ctstate ESTABLISHED,RELATED"
-      jump "ACCEPT"
-    end
-
-    # Allow SSH
-    simple_iptables_rule "ssh" do
-      chain "system"
-      rule "--proto tcp --dport 22"
+    simple_iptables_rule "system" do
+      rule [ # Allow all traffic on the loopback device
+             "--in-interface lo",
+             # Allow any established connections to continue, even
+             # if they would be in violation of other rules.
+             "-m conntrack --ctstate ESTABLISHED,RELATED",
+             # Allow SSH
+             "--proto tcp --dport 22",
+           ]
       jump "ACCEPT"
     end