add missing mitaka-3 release notes

Change-Id: Ibfc47ebe0652b38dbc708d2a0ab58a4288230228

releasenotes/notes/bug1540395-reason-string-0aad56966007d0e3.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - >
+    [`bug 1540395 <https://bugs.launchpad.net/aodh/+bug/1540395>`_]
+    Fix reason string to properly handle transitions when one sample is outside
+    of defined threshold.

releasenotes/notes/deprecate-nosql-backends-13079883eec7e8e5.yaml

@@ -0,0 +1,7 @@
+---
+deprecations:
+  - >
+    Drop support for NoSQL backends in Aodh. SQL is a prefectly sufficient
+    backend for handling the scope of alarms. To maximise available resources,
+    NoSQL backends are deprecated so developers do not need to worry about
+    adding features to multiple backends.

releasenotes/notes/fix-empty-statistics-3852da99b1c0b297.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - >
+    [`bug 1539069 <https://bugs.launchpad.net/aodh/+bug/1539069>`_]
+    Fix to handle scenario where no valid statistics exist for specified
+    period.