Documentation: Add documentation about the release process

It's originally written by Martin who graciously allowed to me rework it
a bit and push it into coreboot's documentation.

Change-Id: I14938d678e4620abec7ed5f0d35dddaf00edda6d
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/30082
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/releases/index.md

@@ -10,6 +10,12 @@ Release notes for previous releases
 * [4.7 - January 2018](coreboot-4.7-relnotes.md)
 * [4.8 - May 2018](coreboot-4.8.1-relnotes.md)
 
+The checklist contains instructions to ensure that a release covers all
+important things and provides a reliable format for tarballs, branch
+names etc.
+
+* [checklist](checklist.md)
+
 Upcoming release
 ----------------