Add example of rolling update using a percentage.

docsite/rst/playbooks_delegation.rst

@@ -33,6 +33,18 @@ use case, you can define how many hosts Ansible should manage at a single time b
 In the above example, if we had 100 hosts, 3 hosts in the group 'webservers'
 would complete the play completely before moving on to the next 3 hosts.
 
+The ''serial'' keyword can also be specified as a percentage, which will be applied to the total number of hosts in a
+play, in order to determine the number of hosts per pass::
+
+    - name: test play
+      hosts: websevers
+      serial: "30%"
+
+If the number of hosts does not divide equally into the number of passes, the final pass will contain the remainder.
+
+.. note::
+     No matter how small the percentage, the number of hosts per pass will always be 1 or greater.
+
 .. _maximum_failure_percentage:
 
 Maximum Failure Percentage