docs: Add changelog (fixes #2)

As agreed, written manually, not from commit messages (https://keepachangelog.com/en/1.0.0/#bad-practices).

.chglog/CHANGELOG.tpl.md

+# Changelog
+{{ range .Versions }}
+## {{ if .Tag.Previous }}[{{ .Tag.Name }}]({{ $.Info.RepositoryURL }}/compare/{{ .Tag.Previous.Name }}...{{ .Tag.Name }}){{ else }}{{ .Tag.Name }}{{ end }} ({{ datetime "2006-01-02" .Tag.Date }})
+
+{{ range .CommitGroups -}}
+### {{ .Title }}
+
+{{ range .Commits -}}
+* {{ .Subject }}
+{{ end }}
+{{ end -}}
+
+{{- if .NoteGroups -}}
+{{ range .NoteGroups -}}
+### {{ .Title }}
+
+{{ range .Notes }}
+{{ .Body }}
+{{ end }}
+{{ end -}}
+{{ end -}}
+{{ end -}}