Update CHANGELOG.md

fixing coverage bug in coverage2cytosine when `--gc_context` was specified

CHANGELOG.md

@@ -12,6 +12,10 @@
 
 Added scripts for merging coverage files (e.g. for when R1 and R2 had been run in single-end mode) 
 
+### coverage2cytosine
+
+- set threshold reads to 1 (if it was 0) for `--gc_context` as intended and mentioned in the help text. Fixes [#621](https://github.com/FelixKrueger/Bismark/issues/621)
+
 ## Changelog for Bismark v0.24.1 (release on 29 May 2023)
 
 - Added new [documentation website](http://felixkrueger.github.io/Bismark/), built using [Material for Mkdocs](https://squidfunk.github.io/mkdocs-material/). Thanks to @ewels for a great (late-night) effort to break up and restructure what had become a fairly unwieldy monolithic beast