add note re. ram_mb default

Author: sjpb

README.md

@@ -18,7 +18,7 @@ Role Variables
   Otherwise, `groups` can be omitted and the following attributes can be defined in the partition object:
   * `name`: The name of the nodes within this group.
   * `cluster_name`: Optional.  An override for the top-level definition `openhpc_cluster_name`.
-  * `ram_mb`: Optional.  The physical RAM available in each server of this group ([slurm.conf](https://slurm.schedmd.com/slurm.conf.html) parameter `RealMemory`).
+  * `ram_mb`: Optional.  The physical RAM available in each server of this group ([slurm.conf](https://slurm.schedmd.com/slurm.conf.html) parameter `RealMemory`). This is set to the Slurm default of `1` if not defined.
 
   For each group (if used) or partition there must be an ansible inventory group `<cluster_name>_<group_name>`. All nodes in this inventory group will be added to the group/partition. Nodes may have arbitrary hostnames but these should be lowercase to avoid a mismatch between inventory and actual hostname.