docs(CHANGELOG): Add v0.3.1 section

CHANGELOG.md

@@ -4,7 +4,13 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## [Unreleased](https://github.com/Xint0/banxico-php/compare/v0.3.0...master)
+## [Unreleased](https://github.com/Xint0/banxico-php/compare/v0.3.1...master)
+
+## [v0.3.1](https://github.com/Xint0/banxico-php/compare/v0.3.0...v0.3.1)
+
+### Changed
+
+- Do not force exclusion of `symfony/polyfill-php73`.
 
 ## [v0.3.0](https://github.com/Xint0/banxico-php/compare/v0.2.0...v0.3.0)