Add release notes page for version 0.8.0

Patrole 0.8.0 has been released [1].
This commit adds a new page for 0.8.0 release notes.

[1] https://review.opendev.org/#/c/703523/

Change-Id: I61333ba564d6f067dd8063fd89847ac58ed93d1d
diff --git a/releasenotes/source/index.rst b/releasenotes/source/index.rst
index 543561f..32f74ec 100644
--- a/releasenotes/source/index.rst
+++ b/releasenotes/source/index.rst
@@ -6,6 +6,7 @@
    :maxdepth: 1
 
    unreleased
+   v0.8.0
    v0.7.0
    v0.6.0
    v0.5.0