add changelog entry

changelog.txt

@@ -30,6 +30,7 @@ Template for new versions:
 - `control-panel`: new commandline interface for control panel functions
 - `uniform-unstick`: (reinstated) force squad members to drop items that they picked up in the wrong order so they can get everything equipped properly
 - `gui/reveal`: temporarily unhide terrain and then automatically hide it again when you're ready to unpause
+- `gui/biomes`: visualize and inspect biome regions on the map
 
 ## New Features
 - `uniform-unstick`: add overlay to the squad equipment screen to show a equipment conflict report and give you a one-click button to fix