Add missing option to readme (#446)

* Update readme for missing option.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: caffeinepills

README.md

@@ -62,6 +62,9 @@ The following configuration options are accepted:
   `module.for.Class`). If `False`, just the class name displays (e.g. `Class`)
 - `always_document_param_types` (default: `False`): If `False`, do not add type info for undocumented parameters. If
   `True`, add stub documentation for undocumented parameters to be able to add type info.
+- `always_use_bars_union ` (default: `False`): If `True`, display Union's using the | operator described in PEP 604.
+   (e.g `X` | `Y` or `int` | `None`). If `False`, Unions will display with the typing in brackets. (e.g. `Union[X, Y]`
+   or `Optional[int]`)
 - `typehints_document_rtype` (default: `True`): If `False`, never add an `:rtype:` directive. If `True`, add the
   `:rtype:` directive if no existing `:rtype:` is found.
 - `typehints_use_rtype` (default: `True`): Controls behavior when `typehints_document_rtype` is set to `True`. If