Update CHANGELOG.md

Author: amartinhuertas

CHANGELOG.md

@@ -5,6 +5,12 @@ 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] 
+
+### Added 
+
+- Added an MPI ibarrier-based (supposedly scalable) algorithm to find rcv neighbours in a sparse all-to-all communication graph given the snd neighbors. We left the previous non-scalable algorithm as default (based on gather-scatter) until we have experimental evidence on the relative performance and scalability of the former with respect to the latter and for which core ranges.
+
 ## [0.3.2] - 2023-05-10
 
 ### Fixed