Documentation/releases: Add 4.15 release notes template

Change-Id: I52bd1ee6b297ba08e335f5c65941b09f14689a00
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/54031
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

Documentation/releases/coreboot-4.15-relnotes.md

@@ -0,0 +1,16 @@
+Upcoming release - coreboot 4.15
+================================
+
+The 4.15 release is planned for October 2021.
+
+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.
+
+Significant changes
+-------------------
+
+### Add significant changes here