> I suggest you start with an ELPA package. If it turns out to be
> popular, we can later include it in Emacs.

Ok, I'll call it adjust-parens.

In ELPA, 'make archive' expects a package ChangeLog, even while
existing packages don't have one.

  $ make archive
  rm -r archive-tmp
  rm: cannot remove `archive-tmp': No such file or directory
  make: [archive-tmp] Error 1 (ignored)
  mkdir -p archive-tmp
  cp -a packages/. archive-tmp/packages
  make  process-archive
  make[1]: Entering directory `/redacted/linux/boreilly/sw/elpa'
  # FIXME, we could probably speed this up significantly with
  # rules like "%.tar: ../%/ChangeLog" so we only rebuild the packages
  # that have indeed changed.
  cd archive-tmp/packages; \
            emacs --batch -l
/redacted/linux/boreilly/sw/elpa/admin/archive-contents.el \
              -f batch-make-archive
  Skipping non-package file README
  Wrote /home/boreilly/l/sw/elpa/archive-tmp/packages/ack/ack-pkg.el
  Error in adaptive-wrap: (file-error "Opening input file" "no such file or
directory"
"/home/boreilly/l/sw/elpa/archive-tmp/packages/adaptive-wrap/ChangeLog")
  make[1]: *** [process-archive] Error 255
  make[1]: Leaving directory `/redacted/linux/boreilly/sw/elpa'
  make: *** [archive] Error 2

I fixed it with this patch (git diff -w):

diff --git a/admin/archive-contents.el b/admin/archive-contents.el
index 2d588e9..640c285 100644
--- a/admin/archive-contents.el
+++ b/admin/archive-contents.el
@@ -206,6 +206,7 @@ Rename DIR/PKG.el to PKG-VERS.el, delete DIR, and
return the descriptor."
       (re-search-backward "^;;;.*ends here")
       (re-search-backward "^(provide")
       (skip-chars-backward " \t\n")
+      (when (file-readable-p cl)
         (insert "\n\n;;;; ChangeLog:\n\n")
         (let* ((start (point))
                (end (copy-marker start t)))
@@ -213,7 +214,7 @@ Rename DIR/PKG.el to PKG-VERS.el, delete DIR, and
return the descriptor."
           (goto-char end)
           (unless (bolp) (insert "\n"))
           (while (progn (forward-line -1) (>= (point) start))
-          (insert ";; ")))
+            (insert ";; "))))
       (set (make-local-variable 'backup-inhibited) t)
       (basic-save-buffer)               ;Less chatty than save-buffer.
       (kill-buffer)))

Shall I commit this?

After that, list-packages shows the new adjust-parens package in the
local ELPA archive.

Here are some ELPA README redlines:

diff --git a/README b/README
index 097e430..3d01912 100644
--- a/README
+++ b/README
@@ -29,9 +29,9 @@ each package.

 *** Add a multi-file package as a directory, packages/NAME.

-*** Commit your changes the usual way ("bzr add", "bzr commit", etc).
+*** Commit your changes the usual way ("git add", "git commit", etc).

-Changes in the Bzr repository do not immediately propagate to the
+Changes in the Git repository do not immediately propagate to the
 user-facing archive (what users see when they do `M-x list-packages').
 That is done by deploying the archive.

@@ -67,7 +67,7 @@ and adds them to the archive.

 ** To access a deployed archive

-To access the archive via HTPP, have a symlink (say) /var/www/packages
+To access the archive via HTTP, have a symlink (say) /var/www/packages
 pointing to DEST/packages, and set up Emacs with

   (setq package-archives '(("new-elpa" . "http://foo.com/packages")))

The ELPA README mentions having a "site" build target, but:

  $ make site
  make: *** No rule to make target `site'.  Stop.

If the site target to Make is indeed gone, I'll remove wording from
the README about it too.

With no objections, I'll commit the above diffs and the attached
adjust-parens.el file to ELPA.