Help with custom Directives

Hello to the community, I would like to know if it is not possible to define custom directives, according to the documentation of the **GraphQLSchema** class if possible, by explicitly declaring them in the schema constructor like this:

```python
MyAppSchema = GraphQLSchema(
...
directives=specified_directives.extend([MyCustomerDirective]),
)
```

Start with the GraphQLSchema documentation and the schema constructor example in the issue. Check how specified_directives and MyCustomerDirective are represented and whether custom directives are covered by existing tests or examples. Done would require a clearly defined, supported behavior and corresponding documentation or tests, but this issue does not name files or a concrete change.

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