Documentation/releases: Update for 4.13

Fill in some blanks for 4.13, mark it done, add template for 4.14.
Also update the list of vboot supported boards.

Change-Id: Ie593efe515136a3b06620db6f0dbe3da00df7e9b
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47801
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

Documentation/releases/coreboot-4.14-relnotes.md

@@ -0,0 +1,16 @@
+Upcoming release - coreboot 4.14
+================================
+
+The 4.14 release is planned for May 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