Update CHANGELOG.md

Author: danawillow

CHANGELOG.md

@@ -10,6 +10,7 @@ IMPROVEMENTS:
 * compute: Add disabled property to `google_compute_firewall` [GH-1536]
 * compute: Add support for custom request headers in `google_compute_backend_service` [GH-1537]
 * compute: Add support for `ssl_policy` to `google_compute_target_ssl_proxy` [GH-1568]
+* compute: Add support for `version`s in instance group manager [GH-1499]
 * cloudbuild: Use the project defined in `trigger_template` when creating a `google_cloudbuild_trigger` [GH-1556]
 * kms: Add basic update for `google_kms_crypto_key` resource [GH-1511]
 * project: Use default provider project for `google_project_services` if project field is empty [GH-1553]