Add simple playbook task for upgrading servers

We want to treat our servers as cattle; hopefully when this is fleshed out a bit more, it can accomplish the job without too many casualties.

README.md

@@ -136,8 +136,9 @@ playbook can be used:
     ansible-playbook playbooks/tasks/upgrade-servers.yml [-l SUBSET]
 
 It runs `pacman -Syu` on the targeted hosts in batches and then reboots them.
-If any server fails to reboot successfully, the rolling upgrade stops and
-further batches are cancelled.
+If any server fails to reboot successfully, the rolling update stops and
+further batches are cancelled. To display a list of packages updated on each
+host, you can pass the `--diff` option to ansible-playbook.
 
 Using this update method, `.pacnew` files are left unmerged which is OK for
 most configuration files that are managed by Ansible. However, care must be