Documentation: Add template for deprecation notices

Change-Id: Ia2cc4f799804c7d56db572823246c487cd19a726
Signed-off-by: Patrick Georgi <patrick@coreboot.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/59677
Reviewed-by:  Felix Singer <felixsinger@posteo.net>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/releases/index.md

@@ -23,6 +23,11 @@ names etc.
 
 * [checklist](checklist.md)
 
+For release related communications consider using a template so everything
+important is taken care of.
+
+* [templates](templates.md)
+
 Upcoming release
 ----------------