add about-note to examples/host.yml

Co-authored-by: Slavi Pantaleev <slavi@devture.com>

examples/host.yml

@@ -1,4 +1,7 @@
 ---
+
+# This is a host file for usage with the `ansible-all-hosts.sh` script,
+# which runs Ansible against a bunch of hosts, each with its own `sudo` password.
 matrix_servers:
   hosts:
     matrix.<your domain>: