add CHANGELOG.md for v0.1.0 release

Initial release. Drop-in replacement for fatih/structs with modern Go improvements.

Highlights

• Core Struct type with functional options (WithTag)
• Conversion: Map(), Values(), Names(), Fields()
• Field type with inspection and mutation methods
• Tag parsing: omitempty, -, omitnested, string
• Compatibility layer: FillMap() for fatih/structs migration
• Error returns instead of panics
• Nil pointer safety for embedded/nested structs
• 96.5% test coverage

Install

go get github.com/agentine/structify@v0.1.0