Add unit tests for authentication and webhook parsing

- Implement tests for basic and bearer authentication headers in `test_auth.py`.
- Create tests for the `EasybillWebhookParser` in `test_webhooks.py`, covering JSON and form-encoded payloads, as well as a generic parse and acknowledgement method.

README.md

@@ -0,0 +1,34 @@
+# easybill client
+
+Python client for the easybill REST API with a generated API layer, Pydantic-based convenience models, and webhook parsing helpers for middleware integration.
+
+## Current status
+
+The initial implementation is in place and includes:
+
+- a project scaffold with packaging and tests
+- sync and async wrapper clients
+- authentication helpers for bearer and basic auth
+- a webhook parser for JSON and form payloads
+- a reproducible generation script based on the provided Swagger specification
+
+## Development
+
+Install the local project dependencies and run the tests:
+
+```powershell
+python -m pytest -q
+```
+
+Generate the raw clients from the API description:
+
+```powershell
+python scripts/generate_client.py --mode both
+```
+
+## Structure
+
+- `src/easybill_client`: public package and middleware-friendly helpers
+- `tests`: focused verification for auth and webhook behavior
+- `scripts/generate_client.py`: generation entrypoint for the raw REST layer
+- `generated`: generated sync and async clients derived from the Swagger description