docs: add a changelog and the release notes flow

CHANGELOG.md

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+# Changelog
+
+What each release brings, newest first. The [GitHub releases](https://github.com/LoonixTools/plasma-face-unlock/releases) add every commit that went into it.
+
+## v1.0.0
+
+_2026-09-23_
+
+Welcome to the very first release of plasma-face-unlock! It brings Face ID to KDE Plasma: look at the screen and it unlocks. The lock screen, sudo and admin prompts can all take your face instead of a password.
+
+<p align="center">
+  <img width="480" alt="The bubble drops down over the lock screen, finds the face and shows a green tick" src="https://raw.githubusercontent.com/LoonixTools/plasma-face-unlock/v1.0.0/res/screenshots/unlock.webp">
+</p>
+
+### Highlights
+
+- Unlock the lock screen with your face
+- sudo and admin prompts
+- A bubble like Face ID
+- Photos do not get in
+- Packages for Arch, Fedora, Debian and Kubuntu