Tracking issue of RFC-44: Use git-chglog

[Xuanwo]

RFC: https://github.com/maintainer-org/maintainer/blob/master/docs/rfcs/44-use-git-chglog.md

Steps

• Use git-chglog instead
• Update README
• Add migration plan

[Xuanwo]

git-chglog must have template files and unable to pass template content directly.

// Config for generating CHANGELOG
type Config struct {
	Bin        string // Git execution command
	WorkingDir string // Working directory
	Template   string // Path for template file. If a relative path is specified, it depends on the value of `WorkingDir`.
	Info       *Info
	Options    *Options
}