Deliver important Guix changes to users, please

[znavko]

I was reading this https://git.savannah.gnu.org/cgit/guix.git/log/?qt=grep&q=slim
and found a record '2019-03-28 services: desktop: Switch to GDM'.
This change had made users' configurations unworkable. Update to this commit could bring bothers.
I think it will be the best to have some layer of informative support for updates. Like this:

$ guix system --updates-available
Commit: 1 [two days ago]
Changes: some services are available now

Commit: 2
Changes: [red]Switch to GDM in %desktop-services. If your config is using %desktop-services, you need to check manual pages.[/red]

Commit: 3 (latest)
Changes: gc clear-generations added


$ guix package --updates-available=1week
90 new packages added.
15 packages updated.
New packages:
emacs-evil-lio, llgpl-license, ...
Changed package:
emacs 26.1 => 26.2
...


That will give some knowledge that your cool config might be wrong after update to the latest commit, where dm was switched to gdm.

I think news should be divided to
- 'packages news' defining the period,
- and 'system news', the default period is from your last `guix system reconfigure config.scm` or may be your last generation's date till now,

And sure news should be available from guix command.

There are tools for update guix from substitute servers: local or Internet. There is much info in cgit logs on gnu server... Want to read more with guix.

Would this kind of messages help people to bypass troubles in convenient way?