Add a release summary

CHANGELOG.rst

@@ -7,9 +7,18 @@ Changelog
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 **Date:** TBA
 
-[Title]
-
-[Summary]
+ADIOS2 Joined arrays, API simplification, HDF5 subfiling, TOML backend, wildcard file extensions, Performance & Memory
+
+This release adds support for additional I/O features in all backends, and additionally a completely new TOML backend, useful for openPMD-formatted configuration files in scientific workflows.
+The ADIOS2 backend now supports joined arrays for simplified storage of particle data in parallel, asynchronous I/O, group tables for enhanced support of ADIOS2 I/O steps, optimized attribute aggregation schemes for large-scale setups and more features.
+The HDF5 backend has added support for the "subfiling" virtual file driver intended for I/O performance in large-scale setups, explicit JSON/TOML-based configuration for chunking and independent flushing. The support of exotic datatypes is improved, such as float128 on ARM64/PPC64 and improved support for generically dealing with unknown datatypes.
+The JSON backend supports parallel MPI output into separate subfiles, mainly intended for debugging purposes.
+The ADIOS1 backend, previously deprecated, has been removed in favor of ADIOS2.
+Please consult the upgrade guide for hints on further interacting with old data.
+
+The openPMD-api has been simplified by no longer requiring explicit specification for scalar components via ``RecordComponent::SCALAR`` in its object model.
+The addition of wildcard filename extensions (e.g. ``simData.%E`` or ``simData_%T.%E``) simplifies the generic implementation of backend-independent logic, such as post-processing routines.
+Rank tables can now be used to pass detailed topology information from writer to reader in staging setups.
 
 Changes to "0.15.0"
 ^^^^^^^^^^^^^^^^^^^