nixos/timekpr: init at 0.5.8

Co-authored-by: Matt Sturgeon <matt@sturgeon.me.uk>

nixos/doc/manual/configuration/user-mgmt.chapter.md

@@ -152,3 +152,16 @@ original files are by default stored in `/var/lib/nixos`.
 Userborn implements immutable users by re-mounting the password files
 read-only. This means that unlike when using the Perl script, trying to add a
 new user (e.g. via `useradd`) will fail right away.
+
+## Restrict usage time {#sec-restrict-usage-time}
+
+[Timekpr-nExT](https://mjasnik.gitlab.io/timekpr-next/) is a screen time managing application that helps optimizing time spent at computer for your subordinates, children or even for yourself.
+
+You can enable it via:
+
+```nix
+{ services.timekpr.enable = true; }
+```
+
+This will install the `timekpr` package and start the `timekpr` service.
+You can then use the `timekpra` application to configure time limits for users.