Update README.md

README.md

 # Cisco AsyncOS (IronPort)
 
+**Note: this package is for CheckMK version 2.x. For other versions see the corresponding branch.**
+
 Collection of Cisco AsyncOS checks
 *  *cisco_asyncos_fan*: monitors the appliance fans
 *  *cisco_asyncos_raid*: monitors the appliance RAID drives
@@ -108,4 +110,4 @@ Sample output
 Sample output
 
 ![sample output](/doc/sample_updates.png?raw=true)
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