Update master for stable/zed

Add file to the reno documentation build to show release notes for
stable/zed.

Use pbr instruction to increment the minor version number
automatically so that master versions are higher than the versions on
stable/zed.

Sem-Ver: feature
Change-Id: I58401373b25d20594edd1dd443c29fa7453731b3

releasenotes/source/index.rst

@@ -6,6 +6,7 @@
    :maxdepth: 1
 
    unreleased
+   zed
    yoga
    xena
    wallaby

releasenotes/source/zed.rst

@@ -0,0 +1,6 @@
+========================
+Zed Series Release Notes
+========================
+
+.. release-notes::
+   :branch: stable/zed