Add reno noting recent changes

Change-Id: I7341292079aa4bdb2c36a3a1103d94ea04658ecd

releasenotes/notes/add-client-server-overhaul-c5b6f8c01126b4a3.yaml

@@ -0,0 +1,24 @@
+---
+features:
+  - |
+    Changes the design of the VirtualBMC tool. Instead of forking the
+    ``vbmc`` command-line tool to become a daemon and serve a single
+    libvirt domain, the ``vbmcd`` master process and ``vbmc`` command-line
+    client have been introduced. These client-server tools communicate
+    over the ZeroMQ queue. The ``vbmcd`` process is responsible for
+    herding its children, each child still serves a single libvirt
+    domain.
+  - |
+    The ``vbmc start`` command now supports multiple domains.
+upgrade:
+  - |
+    It is advised to invoke ``vbmcd`` master process on system boot,
+    perhaps by a systemd unit file.
+deprecations:
+  - |
+    Deprecates automatically starting up the ``vbmcd`` daemon process if
+    it is not running. This backward-compatibility feature will be removed
+    in the OpenStack Stein release.
+security:
+  - |
+    Hardens PID file creation to prevent the symlink attack.