Add release note for plugin directory

Change-Id: I620c13c7a83f17ae13b03e9c2a52864c154fbdf8

releasenotes/notes/plugin-directory-55861f4098813ba6.yaml

@@ -0,0 +1,15 @@
+---
+features:
+  - |
+    Introduced neutron_lib.plugins.directory to get references
+    for loaded plugins in a neutron server process. For example:
+
+    .. code-block:: python
+
+       from neutron_lib import constants
+       from neutron_lib.plugins import directory
+
+       core_plugin = directory.get_plugin()
+       l3_plugin = directory.get_plugin(constants.L3)
+
+    For more examples, see: https://review.openstack.org/#/c/386845/