Add a note that this repository not used for systemd ≥ 256

[gertvdijk]

It took me more than a few minutes to find out this repository is not updated for branches ≥ 256. This is explained in the main repo changelog with systemd/systemd@40d637b but not here in systemd-stable.

Could this be highlighted somewhere without having to find it out the hard way? in README/NEWS or a pinned issue for example? Thanks.

E.g.:

This repository is deprecated. For systemd releases ≥ 256, please see the main repository for stable releases. systemd-stable is still maintained for the lifetime of the ≤ 255 branches.

[bluca]

Noted in the 'about' field