Add release note for Redfish exporter

Author: markgoddard

releasenotes/notes/redfish-exporter-36cbdabf103ca93d.yaml

@@ -0,0 +1,7 @@
+---
+features:
+  - |
+    Adds support for deploying a Prometheus Redfish exporter container on the
+    seed. This can be used to query the overcloud BMCs via their redfish
+    interfaces to produce various metrics relating to the hardware, and system
+    health.