Add release notes page for version 0.12.0

Patrole 0.12.0 has been released [1].
This commit adds a new page for 0.12.0 release notes.

[1] https://review.opendev.org/c/openstack/releases/+/783115

Change-Id: Ibbb1000d633372e2ca37b306fec3c284059905ba

releasenotes/source/index.rst

@@ -6,6 +6,7 @@ Patrole Release Notes
    :maxdepth: 1
 
    unreleased
+   v0.12.0
    v0.11.0
    v0.10.0
    v0.9.0

releasenotes/source/v0.12.0.rst

@@ -0,0 +1,6 @@
+=====================
+v0.12.0 Release Notes
+=====================
+
+.. release-notes:: 0.12.0 Release Notes
+   :version: 0.12.0