docs: add CHANGELOG and release announcement for v3.0.0

Author: mayurrawte

ANNOUNCEMENT.md

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+# shipthisapi-python v3.0.0 Release
+
+We're excited to announce the release of **shipthisapi-python v3.0.0** - now with full async support!
+
+## What's New
+
+### Async-First Design
+The library has been completely rewritten to support async/await, making it perfect for modern Python applications like FastAPI, aiohttp, and other async frameworks.
+
+```python
+import asyncio
+from ShipthisAPI import ShipthisAPI
+
+async def main():
+    client = ShipthisAPI(
+        organisation="your_org_id",
+        x_api_key="your_api_key"
+    )
+
+    await client.connect()
+
+    # Fetch shipments
+    shipments = await client.get_list("sea_shipment")
+
+    # Update a document
+    await client.patch_item(
+        "fcl_load",
+        "68a4f906743189ad061429a7",
+        update_fields={"container_no": "CONT123"}
+    )
+
+asyncio.run(main())
+```
+
+### Custom Headers Support
+Server-to-server authentication is now supported via custom headers:
+
+```python
+client = ShipthisAPI(
+    organisation="org_id",
+    custom_headers={
+        "authorization": "Bearer your_token",
+        # ... other custom headers
+    }
+)
+```
+
+### Simplified Field Updates
+`patch_item` is now the recommended way to update document fields. It provides:
+- Full field validation
+- Workflow triggers
+- Audit logging
+- Business logic execution
+
+## Breaking Changes
+
+1. **All methods are now async** - You must use `await` when calling API methods
+2. **`requests` replaced with `httpx`** - The library now uses `httpx` for HTTP calls
+3. **Removed deprecated methods**:
+   - `webhook_sync` - Use `patch_item` instead
+   - `webhook_update` - Use `patch_item` instead
+
+## Installation
+
+```bash
+pip install shipthisapi-python==3.0.0
+```
+
+Or update your requirements:
+```
+shipthisapi-python>=3.0.0
+```
+
+## Migration Guide
+
+See [CHANGELOG.md](./CHANGELOG.md) for a detailed migration guide from v2.x to v3.x.
+
+## Questions or Issues?
+
+- Open an issue on [GitHub](https://github.com/shipthisco/shipthisapi-python/issues)
+- Contact support at support@shipthis.co

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+## [3.0.0] - 2025-02-06
+
+### Breaking Changes
+- **Async-first**: All API methods are now async and require `await`
+- Replaced `requests` library with `httpx` for async HTTP support
+- Removed `webhook_sync` and `webhook_update` methods (use `patch_item` instead)
+
+### Added
+- Full async/await support using `httpx`
+- `custom_headers` parameter for server-to-server authentication
+- `create_reference_linked_field` method
+- Per-request header override support
+
+### Changed
+- `x_api_key` is now optional (can use `custom_headers` for auth)
+- `patch_item` is now the recommended method for updating document fields
+- Updated all method signatures to be async
+
+### Migration Guide
+
+**Before (v2.x):**
+```python
+from ShipthisAPI import ShipthisAPI
+
+client = ShipthisAPI(organisation="org_id", x_api_key="key")
+client.connect()
+items = client.get_list("shipment")
+client.patch_item("fcl_load", doc_id, {"status": "done"})
+```
+
+**After (v3.x):**
+```python
+import asyncio
+from ShipthisAPI import ShipthisAPI
+
+async def main():
+    client = ShipthisAPI(organisation="org_id", x_api_key="key")
+    await client.connect()
+    items = await client.get_list("shipment")
+    await client.patch_item("fcl_load", doc_id, {"status": "done"})
+
+asyncio.run(main())
+```
+
+## [2.2.0] - 2025-02-06
+
+### Added
+- `custom_headers` parameter for overriding default headers
+- `create_reference_linked_field` method
+- Per-request header override in `_make_request`
+
+### Changed
+- `x_api_key` is now optional
+- Enhanced `patch_item` documentation
+
+## [2.1.0] - 2025-01-15
+
+### Added
+- `primary_workflow_action` method for workflow transitions
+- `secondary_workflow_action` method for sub-status changes
+- `bulk_edit` method for batch updates
+
+## [2.0.0] - 2024-12-01
+
+### Added
+- Complete rewrite with better error handling
+- `ShipthisAPIError`, `ShipthisAuthError`, `ShipthisRequestError` exceptions
+- Comprehensive CRUD operations
+- Workflow operations
+- Report views
+- Third-party integrations (currency, places)
+- Conversation methods
+- File upload support
+
+## [1.0.0] - 2024-06-01
+
+### Added
+- Initial release
+- Basic API client functionality