 Update CHANGELOG.md (#4137)
 Update CHANGELOG.md (#4137)

motivation: keep change log up to date 

changes:
* add 5.7 section
* document user facing API changes in #4119
apple/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager