Added CHANGELOG for partitionedsolvers

Author: fverdugo

PartitionedSolvers/CHANGELOG.md

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+
+# Changelog
+
+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).
+
+## [0.1.0] - Unreleased
+
+### Added
+
+- Basic smoothed aggregation algebraic multigrid preconditioner for the Laplace problem