CHANGES: rename to CHANGES.md, no longer generated

- scripts/log2changes.pl was not included in release tarballs, which broke
  reproducible builds

- since log2changes uses git to generate the contents, it makes it difficult
  to generate the same contents later (it would need to be fixed)

- the CHANGES file has outlived its purpose. the main changes are in the
  RELEASE-NOTES, the rest are better tracked directly using git or on GitHub

- put a fixed CHANGES.md in there instead pointing out where the info lives
  now

Closes #14331

CHANGES.md

@@ -0,0 +1,12 @@
+<!--
+Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+
+SPDX-License-Identifier: curl
+-->
+
+In a release tarball, check the RELEASES-NOTES file for what was done in the
+most recent release. In a git check-out, that file mentions changes that have
+been done since the previous release.
+
+See the online [changelog](https://curl.se/changes.html) for the edited and
+human readable version of what has changed in different curl releases.