CHANGELOG: add note about config writing changes

CHANGELOG.md

@@ -6,6 +6,10 @@ v0.26 + 1
 * Improved `p_unlink` in `posix_w32.c` to try and make a file writable
   before sleeping in the retry loop to prevent unnecessary calls to sleep.
 
+* Writing to a configuration file now preserves the case of the key given by the
+  caller for the case-insensitive portions of the key (existing sections are
+  used even if they don't match).
+
 ### API additions
 
 * `git_remote_create_detached()` creates a remote that is not associated