Add release notes page for version 0.6.0

Patrole 0.6.0 has been released [1].
This commit adds a new page for 0.6.0 release notes.

[1] https://review.opendev.org/#/c/672485/

Change-Id: Ife0ac2e4a38d96ea312e6c410f85b4e0a97c2f4a
diff --git a/releasenotes/source/index.rst b/releasenotes/source/index.rst
index ca4384e..a51c1e4 100644
--- a/releasenotes/source/index.rst
+++ b/releasenotes/source/index.rst
@@ -6,6 +6,7 @@
    :maxdepth: 1
 
    unreleased
+   v0.6.0
    v0.5.0
    v0.4.0
    v0.3.0