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/bt-volume-step/releases) add every commit that went into it.
+
+## v1.0.0
+
+_2026-08-16_
+
+Welcome to the very first release of bt-volume-step! It gives your Bluetooth headphones and speakers clean volume steps.
+
+AirPods Pro only know 16 volume steps, so every swipe moves the volume by 6.25 %, and your desktop walks through numbers like these:
+
+```
+6 · 13 · 19 · 25 · 31 · 38 · 44 · 50 · 56 · 63 · 69 · 75 · 81 · 88 · 94 · 100
+```
+
+### Highlights
+
+- One swipe, one clean step
+- Learns every device on its own
+- Follows Plasma