Update release_notes.rst

Co-authored-by: Johannes HAMPP <42553970+euronion@users.noreply.github.com>

Author: fneum

docs/release_notes.rst

@@ -16,7 +16,7 @@ Upcoming Release
 ..   The features listed below are not released yet, but will be part of the next release! 
 ..   To use the features already you have to use the ``master`` branch.
 
-* Changed currency year from 2020 to 2025 for all cost assumptions.
+* Changed currency year from 2020 to 2025 for all cost assumptions using updated Eurostat inflation rates.
 
 * Update fossil fuel (gas, oil, lignite) and nuclear fuel (nuclear, uranium) costs based on TYNDP 2024 scenarios data.