Update README.md

Author: tpherndon

README.md

@@ -71,6 +71,11 @@ Options:
  do not monitor the number of nodes in the cluster.
  -p PORT, --port=PORT TCP port to probe. The ElasticSearch API should be
  listening here. Defaults to 9200.
+ -y YELLOW_CRITICAL, --yellow-critical=TRUE
+ Instead of issuing a 'warning' for a yellow cluster
+ state, issue a 'critical' alert. Allows for greater
+ control of alerting for clusters that may be of 
+ greater sensitivity (or fragility). Defaults to False.
 
 ```