release notes file

[Mark Johnston]

Hi,

Today we add a Relnotes tag to commits that warrant a release note.
My impression is that it doesn't work so well: if a committer forgets
or doesn't know to add one there's no way to amend the commit message
(same for MFCs), and a commit message isn't a convenient place to write
the text of a release note.  I would like to propose adding a top-level
RELNOTES file instead, which like UPDATING would document notes for
specific commits.  It would be truncated every time the head branch is
forked, and changes to it would be MFCed.  This fixes the
above-mentioned problems and would hopefully reduce the amount of time
needed by re@ to compile release notes.

For example:

Index: RELNOTES
===================================================================
--- RELNOTES	(nonexistent)
+++ RELNOTES	(working copy)
@@ -0,0 +1,8 @@
+Release notes for FreeBSD 13.0.
+
+r349286:
+	swapon(8) can now erase a swap device immediately before
+	enabling it, similar to newfs(8)'s -E option.  This behaviour
+	can be specified by adding -E to swapon(8)'s command-line
+	parameters, or by adding the "trimonce" option to a swap
+	device's /etc/fstab entry.

What do folks think?