Add change to changelog

CHANGELOG.rst

 Unreleased changes
 ------------------
 
-* Python environment is not version tracked until now
+* Add decay option to the runner script
 
 
-Version 0
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-
-0.4.0 / 2020-11-09
-~~~~~~~~~~~~~~~~~~
-* Docker: Resized pyhtia/jetset output particle vector
-
-0.3.0 / 2020-09-05
-~~~~~~~~~~~~~~~~~~
-* Docker: Fixed environment variables 
-
-0.2.0 / 2020-09-03
-~~~~~~~~~~~~~~~~~~
-* Docker: GiBUU compiled with RootTuple for root file output
-
-0.1.0 / 2020-01-21
-~~~~~~~~~~~~~~~~~~
-* Docker: First GiBUU docker environment