Add note regarding groups value

Instances are filtered on group name, not group id

Author: Chris Zelenak

README.md

@@ -57,7 +57,7 @@ ec2 discovery allows to use the ec2 APIs to perform automatic discovery (similar
 
 The following are a list of settings (prefixed with `discovery.ec2`) that can further control the discovery:
 
-* `groups`: Either a comma separated list or array based list of (security) groups. Only instances with the provided security groups will be used in the cluster discovery.
+* `groups`: Either a comma separated list or array based list of (security) groups. Only instances with the provided security groups will be used in the cluster discovery. (NOTE: You should provide the group NAME, not the group ID.)
 * `host_type`: The type of host type to use to communicate with other instances. Can be one of `private_ip`, `public_ip`, `private_dns`, `public_dns`. Defaults to `private_ip`.
 * `availability_zones`: Either a comma separated list or array based list of availability zones. Only instances within the provided availability zones will be used in the cluster discovery.
 * `any_group`: If set to `false`, will require all security groups to be present for the instance to be used for the discovery. Defaults to `true`.