docs/releases: Add 4.21 release notes template

Change-Id: Ibb4eaba6be088e20c76a8329847148c1d4ff8c04
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Signed-off-by: Martin Roth <gaumless@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/75187
Reviewed-by: Jason Glenesk <jason.glenesk@amd.corp-partner.google.com>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/releases/coreboot-4.21-relnotes.md

@@ -0,0 +1,51 @@
+Upcoming release - coreboot 4.21
+========================================================================
+
+The 4.21 release is planned for mid-August, 2023
+
+Update this document with changes that should be in the release notes.
+
+* Please use Markdown.
+* See the past few release notes for the general format.
+* The chip and board additions and removals will be updated right
+  before the release, so those do not need to be added.
+* Note that all changes before the release are done are marked upcoming.
+  A final version of the notes are done after the release.
+
+Significant or interesting changes
+----------------------------------
+
+### Add changes that need a full description here
+
+* This section should have full descriptions and can or should have
+  a link to the referenced commits.
+
+Additional coreboot changes
+---------------------------
+
+The following are changes across a number of patches, or changes worth
+noting, but not needing a full description.
+
+* Changes that only need a line or two of description go here.
+
+Platform Updates
+----------------
+
+* To be filled in immediately before the release by the release team
+
+Plans to move platform support to a branch
+------------------------------------------
+
+* Seciton to be filled in or removed after discussion
+
+Statistics from the 4.20 to the 4.21 release
+--------------------------------------------
+
+* To be filled in immediately before the release by the release team
+
+
+Significant Known and Open Issues
+---------------------------------
+
+* To be filled in immediately before the release by the release team
+

Documentation/releases/index.md

@@ -3,7 +3,7 @@
 ## Upcoming release
 
 Please add to the release notes as changes are added:
-* [4.20 - April 2023](coreboot-4.20-relnotes.md)
+* [4.21 - August 2023](coreboot-4.21-relnotes.md)
 
 The [checklist] contains instructions to ensure that a release covers all
 important things and provides a reliable format for tarballs, branch