change module->library, update the example playbook

By default ansible will load libs in library/ which means
you do not need to specify a module path if you are invoking
ansible from the playbooks/ dir.

Added some comments to the rolling example

playbooks/edxapp_rolling_example.yml

-# ansible-playbook -v --user=ubuntu  edxapp_rolling_example.yml -i ./ec2.py  --private-key=/path/to/deployment.pem --module-path /path/to/repo/modules
+# ansible-playbook -v --user=ubuntu  edxapp_rolling_example.yml -i ./ec2.py  --private-key=/path/to/deployment.pem 
+
 - hosts: tag_Group_anothermulti
   serial: 1
   vars_files:
@@ -10,10 +11,18 @@
       ec2_facts: 
     - name: Gathering ELB facts
       local_action: ec2_elb_facts
+# These two modules "ec2_facts" and "ec2_elb_facts" are invoked in the 
+# pre_tasks and give us the $elbs and $ansible_ec2_isntance_id facts 
+# which are variables that can be used in the playbook
     - local_action: command util/elb_reg.py -e {{ ",".join(elbs[ansible_ec2_instance_id]) }} -i {{ ansible_ec2_instance_id }} deregister
+# -e is the list of elbs that the current instances belong to and -i is 
+# the instance "active_elbs" are the elbs that are passed in.
   roles:
     - common
     - nginx 
     - lms
   post_tasks:
     - local_action: command util/elb_reg.py -e {{ ",".join(elbs[ansible_ec2_instance_id]) }} -i {{ ansible_ec2_instance_id }} register
+# Register will pass in the same elb list and the same instance id 
+# to add it back to the pool
+