Add release notes

Author: Ole Markus With

docs/releases/1.19-NOTES.md

@@ -42,6 +42,8 @@ The expiration times vary randomly so that nodes are likely to have their certs
 
 * New command for deleting a single instance: [kops delete instance](/docs/cli/kops_delete_instance/)
 
+* Metrics Server is now available as a configurable addon.
+
 # Breaking changes
 
 * Support for Kubernetes 1.9 and 1.10 has been removed.
@@ -62,6 +64,8 @@ The expiration times vary randomly so that nodes are likely to have their certs
 
 * Support for feature flag `Terraform-0.12` has been deprecated and will be removed in kops 1.20. All generated Terraform HCL2/JSON files will support versions `0.12.26+` and `0.13.0+`.
 
+* The [manifest based metrics server addon](https://github.com/kubernetes/kops/tree/master/addons/metrics-server) has been deprecated in favour of a configurable addon.
+
 # Full change list since 1.18.0 release
 
 ## v1.18.0-alpha.3 to v1.19.0-alpha.1