Add release notes for client registration changes

In patch I902cfdea0af371dfa222a9bbf41edc4ea2765926 I forgot
to add the release notes file, so adding it now as a separate patch.

Change-Id: I4465913fa813971a95ca55fbda6c60312610ac64
diff --git a/releasenotes/notes/plugin-client-registration-enhancements-e09131742391225b.yaml b/releasenotes/notes/plugin-client-registration-enhancements-e09131742391225b.yaml
new file mode 100644
index 0000000..b6391b6
--- /dev/null
+++ b/releasenotes/notes/plugin-client-registration-enhancements-e09131742391225b.yaml
@@ -0,0 +1,12 @@
+---
+features:
+  - |
+    When registering service clients from installed plugins, all registrations
+    are now processed, even if one or more fails. All exceptions encountered
+    during the registration process are recorded.  If at least one exception
+    was encountered, the registration process fails and all interim errors are
+    reported.
+  - |
+    The __repr__ method is now implemented for the base `tempest.Exception`
+    class, its implementation is identical to __str__: it reports the error
+    message merged with input parameters.