Add core metrics to README

Author: bedroge

README.md

@@ -4,6 +4,15 @@ Prometheus collector and exporter for metrics extracted from the [Slurm](https:/
 
 ## Exported Metrics
 
+### State of the Cores
+
+* **Allocated**: cores which have been allocated to a job.
+* **Idle**: cores not allocated to a job and thus available for use.
+* **Other**: cores which are unavailable for use at the moment.
+* **Total**: total number of cores.
+
+[Information extracted from the SLURM **sinfo** command](https://slurm.schedmd.com/sinfo.html)
+
 ### State of the Nodes
 
 * **Allocated**: nodes which has been allocated to one or more jobs.