add change description

Author: eli-bl

.changeset/improved_property_merging.md

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+# Improved property-merging behavior with `allOf`
+
+When using `allOf` to extend a base object type, `openapi-python-client` is now able to handle some kinds of modifications to an existing property that would have previously caused an error:
+
+- Overriding attributes that do not affect validation, such as `description`.
+- Specifying stricter validation for the same type, such as setting a `maxLength` for a string that previously had no max length (or previously had a larger max length), or setting a `pattern` regex for a string that previously had none.
+- Changing a nonspecific numeric type to `int`.
+- Changing a property from `any` to a specific type.
+
+You can now tell `openapi-python-client` to overwrite an existing directory, rather than deleting it yourself before 
+running `generate`.