Add examples of the 'serial' and 'delegate_to' keywords to examples/playbooks.

examples/playbooks/batch_size_control.yml

+# ordinarily, without the 'serial' keyword set, ansible will control all of your machines in a play at once, in parallel.
+# if you want to perform a rolling update, so that each play completes all the way through on a certain number of hosts
+# before moving on to the remaining hosts, use the 'serial' keyword like so:
+
+---
+- hosts: all
+  serial: 3
+
+# now each of the tasks below will complete on 3 hosts before moving on to the next 3, regardless of how many
+# hosts are selected by the "hosts:" line
+
+  tasks:
+
+  - name: ping
+    action: ping
+  - name: ping2
+    action: ping
+
+

examples/playbooks/delegation.yml

+---
+
+# this is an example of how we can perform actions on a given host on behalf of all the hosts
+# in a play.  
+#
+# The two main uses of this would be signalling an outage window for hosts that
+# we are going to start upgrading, or to take a machine out of rotation by talking to a load
+# balancer.   
+#
+# This example cheats by replacing the load balancer script with the 'echo' command,
+# leaving actual communication with the load balancer as an exercise to the reader.  In reality,
+# you could call anything you want, the main thing is that it should do something with 
+# $inventory_hostname
+
+# NOTE: see batch_size_control.yml for an example of the 'serial' keyword, which you almost certainly
+# want to use in this kind of example.  Here we have a mocked up example that does something to
+# 5 hosts at a time
+
+- hosts: all
+  serial: 5
+
+  tasks:
+  
+  - name: take the machine out of rotation
+    action: command echo taking out of rotation $inventory_hostname
+    delegate_to: 127.0.0.1
+
+  - name: do several things on the actual host
+    action: command echo hi mom $inventory_hostname
+  
+  - name: put machine back into rotation
+    action: command echo inserting into rotation $inventory_hostname
+    delegate_to: 127.0.0.1
+