[ci skip] add Mac deps

Author: paulfantom

README.md

@@ -15,6 +15,7 @@ Deploy prometheus [node exporter](https://github.com/prometheus/node_exporter) u
 ## Requirements
 
 - Ansible >= 2.5 (It might work on previous versions, but we cannot guarantee it)
+- gnu-tar on Mac deployer host (`brew install gnu-tar`)
 
 ## Role Variables