Add changelog entry

CHANGELOG.md

@@ -5,10 +5,14 @@ When adding entries, please treat them as if they could end up in a release any
 
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+# 0.18.25
+
+* Fixes an issue in which union members targetting Unit would fail to compile when used as traits (see [#1600](https://github.com/disneystreaming/smithy4s/pull/1600)).
+
 # 0.18.24
 
 * Adds missing nanoseconds in Document encoding of EPOCH_SECOND timestamps
-* Add support for `alloy#jsonUnknown`, allowing structures to capture unknown JSON fields in one of their members. 
+* Add support for `alloy#jsonUnknown`, allowing structures to capture unknown JSON fields in one of their members.
 * Add `getMessage` implementation in `Smithy4sThrowable` which will be overridden in cases where the error structure contains a message field, but otherwise will be used to prevent a useless `null` result when `getMessage` is called.
 
 # 0.18.23