Documentation/releases: Update for 4.12

Fill in some blanks for 4.12, mark it done, add template for 4.13.
Also update the list of vboot supported boards.

Change-Id: Id6b663f13367eb40e66af30aadd33991c8dd635c
Signed-off-by: Patrick Georgi <patrick@georgi.software>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41259
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

Documentation/releases/coreboot-4.13-relnotes.md

@@ -0,0 +1,16 @@
+Upcoming release - coreboot 4.13
+================================
+
+The 4.13 release is planned for November 2020.
+
+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