add missing 6.x release notes link (elastic#1842)

Author: SylvainJuge

docs/release-notes.asciidoc

@@ -5,10 +5,11 @@
 
 All notable changes to this project will be documented here.
 
+* <<release-notes-6.x>>
 * <<release-notes-5.x>>
 * <<release-notes-4.x>>
 * <<release-notes-3.x>>
 * <<release-notes-2.x>>
 * <<release-notes-1.x>>
 
-include::../CHANGELOG.asciidoc[]
+include::../CHANGELOG.asciidoc[]