Update changelog

Author: Rodrigo Bermudez Schettino

changelog.md

@@ -16,5 +16,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
-[unreleased]: https://github.com/rodrigobdz/pdfoptim/compare
-<!-- Example: https://github.com/rodrigobdz/pdfoptim/compare/v1.0.0...HEAD -->
+## [1.0.0] - 2019-04-23
+
+### Added
+
+- Optimize file size of PDFs
+
+[unreleased]: https://github.com/rodrigobdz/pdfoptim/compare/v1.0.0...HEAD
+[v1.0.0]: https://github.com/rodrigobdz/pdfoptim/compare/b30480643d7a74ff1f383674a0f48c735fe73faa...v1.0.0