Documentation/releases: Update 4.15 release notes

Update details for upcoming 4.15 release

Change-Id: Ie4d47456cce38e7ec4329f8cb839167017c7e26b
Signed-off-by: Jason Glenesk <jason.glenesk@amd.corp-partner.google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/58974
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-by:  Felix Singer <felixsinger@posteo.net>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>
diff --git a/Documentation/releases/coreboot-4.15-relnotes.md b/Documentation/releases/coreboot-4.15-relnotes.md
index 184b5de..d1fac52 100644
--- a/Documentation/releases/coreboot-4.15-relnotes.md
+++ b/Documentation/releases/coreboot-4.15-relnotes.md
@@ -1,18 +1,22 @@
 Upcoming release - coreboot 4.15
 ================================
 
-The 4.15 release is planned for November 1st, 2021.
+The 4.15 release is planned for November 5th, 2021.
 
-Since 4.14 there have been more than 2448 new commits by more than 219 developers.
+Since 4.14 there have been more than 2597 new commits by more than 219 developers.
 Of these, over 73 contributed to coreboot for the first time.
 
 Welcome to the project!
 
-
-
 Thank you to all the developers who continue to make coreboot the
 great open source firmware project that it is.
 
+Important Announcement
+----------------------
+We are going to be changing the cadence from every 6 months, to every 3 months.
+That means the 4.16 release will be coming in February, 2022.
+
+
 New mainboards
 --------------
 * Asus p8h61-m_pro_cm6630