Update README.md

Author: MarcinZablocki

README.md

@@ -41,6 +41,11 @@ SSH key is generated each time for the environment in the ./key.pem file.
 
 ### Roles
 
+Roles are set in variables.tf as 
+additional_headnode_roles, additional_worker_roles, additional_storage_roles or additional_role_all
+Additional roles provide ability to install and configure applications defined as Salt states. 
+
+Example roles: 
 - intelmpi: provides configured Intel yum repository and installs IntelMPI distribution
 - openmpi: installs OpenMPI from OL repository
 
@@ -49,6 +54,8 @@ SSH key is generated each time for the environment in the ./key.pem file.
 
 #### Filesystems
 
+Storage role servers will be configured as filesystem nodes, while headnode and worker nodes will act as a clients. 
+
 - GlusterFS (requires storage role)
 
   To use BeeGFS set storage_type to beegfs.