Add CHANGELOG.md + small code layout fix
- Added `on_connect` and `on_disconnect` options to `ariadne.asgi.GraphQL`, enabling developers to run additional initialization and cleanup for websocket connections.
- Updated Starlette dependency to 0.15.
- Added support for multiple keys for GraphQL federations.
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+- Made `Query` type optional in federated schemas.
## 0.13.0 (2021-03-17)
