Add release notes page for v0.4.0

Patrole 0.4.0 has been released [1].
This commit adds a new page for v0.4.0 release notes.

[1] https://review.openstack.org/#/c/592277/

Change-Id: I98519d5f4ff788fee485e735f4c284ea1dd00c28
diff --git a/releasenotes/source/index.rst b/releasenotes/source/index.rst
index ce29994..eb061a4 100644
--- a/releasenotes/source/index.rst
+++ b/releasenotes/source/index.rst
@@ -6,6 +6,7 @@
    :maxdepth: 1
 
    unreleased
+   v0.4.0
    v0.3.0
    v0.2.0
    v0.1.0