Restore info about unix_socket_directories

and make it more precise. (Info about RHEL/Centos 6
can be removed as it is not supported by this module
anymore.)

README.md

@@ -49,8 +49,9 @@ Control the state of the postgresql service and whether it should start at boot
         value: '{{ postgresql_unix_socket_directories | join(",") }}'
       - option: log_directory
         value: 'log'
-
-Global configuration options that will be set in `postgresql.conf`. Note that if you override 'log_directory' with another path, this role will create it for you. 
+Global configuration options that will be set in `postgresql.conf`.
+For PostgreSQL versions older than 9.3 you need to at least override this variable and set the `option` to `unix_socket_directory`.
+If you override the value of `option: log_directory` with another path, relative or absolute, then this role will create it for you. 
 
     postgresql_hba_entries:
       - { type: local, database: all, user: postgres, auth_method: peer }