Reflect EXTRA_ARGS in README.

Related to #9

Update `README.md` to reflect all arguments for `add_zserio_library` function.

* Add `EXTRA_ARGS` argument description to the `add_zserio_library` function section.
* Mention that `EXTRA_ARGS` allows passing additional arguments to the zserio compiler.

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For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/Klebert-Engineering/zserio-cmake-helper/issues/9?shareId=XXXX-XXXX-XXXX-XXXX).

README.md

@@ -117,6 +117,8 @@ Once the `zserio-cmake-helper` target is available, you can use the `add_zserio_
 #     is the entry point of your schema.
 #   TOP_LEVEL_PKG [pkg-name]
 #     Optional top-level namespace for your schema.
+#   EXTRA_ARGS
+#     Allows passing additional arguments to the zserio compiler.
 #
 add_zserio_library(mylib WITH_REFLECTION
   ROOT path/to/mylib-schema