Add UPGRADING guide

UPGRADING.md

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+<!-- 
+Copyright 2020 Google LLC
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    https://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+-->
+
+
+# 2.0.0 Migration Guide
+
+The 2.0 release of the `google-cloud-bigquery-datatransfer` client is a significant
+upgrade based on a [next-gen code generator](https://github.com/googleapis/gapic-generator-python),
+and includes substantial interface changes. Existing code written for earlier versions
+of this library will likely require updates to use this version. This document
+describes the changes that have been made, and what you need to do to update your usage.
+
+If you experience issues or have questions, please file an
+[issue](https://github.com/googleapis/python-bigquery-datatransfer/issues).
+
+
+## Supported Python Versions
+
+> **WARNING**: Breaking change
+
+The 2.0.0 release requires Python 3.6+.
+
+
+## Import Path
+
+> **WARNING**: Breaking change
+
+The library was moved into `google.cloud.bigquery` namespace. Existing imports
+need to be updated.
+
+**Before:**
+```py
+from google.cloud import bigquery_datatransfer
+from google.cloud import bigquery_datatransfer_v1
+```
+
+**After:**
+```py
+from google.cloud.bigquery import datatransfer
+from google.cloud.bigquery import datatransfer_v1
+```
+
+
+## Method Calls
+
+> **WARNING**: Breaking change
+
+Methods that send requests to the backend expect request objects. We provide a script
+that will convert most common use cases.
+
+* Install the library
+
+```py
+python3 -m pip install google-cloud-bigquery-datatransfer
+```
+
+* The script `fixup_datatransfer_v1_keywords.py` is shipped with the library. It expects
+an input directory (with the code to convert) and an empty destination directory.
+
+```sh
+$ scripts/fixup_datatransfer_v1_keywords.py --input-directory .samples/ --output-directory samples/
+```
+
+**Before:**
+```py
+from google.cloud import bigquery_datatransfer
+
+client = bigquery_datatransfer.DataTransferServiceClient()
+
+parent_project = "..."
+transfer_config = {...}
+authorization_code = "..."
+
+response = client.create_transfer_config(
+    parent_project, transfer_config, authorization_code=authorization_code
+)
+```
+
+
+**After:**
+```py
+from google.cloud.bigquery import datatransfer
+
+client = datatransfer.DataTransferServiceClient()
+
+parent_project = "..."
+transfer_config = {...}
+authorization_code = "..."
+
+response = client.create_transfer_config(
+    request={
+        "parent": parent_project,
+        "transfer_config": transfer_config,
+        "authorization_code": authorization_code,
+    }
+)
+```
+
+### More Details
+
+In `google-cloud-bigquery-datatransfer<2.0.0`, parameters required by the API were positional
+parameters and optional parameters were keyword parameters.
+
+**Before:**
+```py
+def create_transfer_config(
+    self,
+    parent,
+    transfer_config,
+    authorization_code=None,
+    version_info=None,
+    service_account_name=None,
+    retry=google.api_core.gapic_v1.method.DEFAULT,
+    timeout=google.api_core.gapic_v1.method.DEFAULT,
+    metadata=None,
+):
+```
+
+In the `2.0.0` release, methods that interact with the backend have a single
+positional parameter `request`. Method docstrings indicate whether a parameter is
+required or optional.
+
+Some methods have additional keyword only parameters. The available parameters depend
+on the [`google.api.method_signature` annotation](https://github.com/googleapis/python-bigquery-datatransfer/blob/master/google/cloud/bigquery_datatransfer_v1/proto/datatransfer.proto#L80)
+specified by the API producer.
+
+
+**After:**
+```py
+def create_transfer_config(
+    self,
+    request: datatransfer.CreateTransferConfigRequest = None,
+    *,
+    parent: str = None,
+    transfer_config: transfer.TransferConfig = None,
+    retry: retries.Retry = gapic_v1.method.DEFAULT,
+    timeout: float = None,
+    metadata: Sequence[Tuple[str, str]] = (),
+) -> transfer.TransferConfig:
+```
+
+> **NOTE:** The `request` parameter and flattened keyword parameters for the API are
+> mutually exclusive. Passing both will result in an error.
+
+
+Both of these calls are valid:
+
+```py
+response = client.create_transfer_config(
+    request={
+        "parent": project_path,
+        "transfer_config": {"foo": "bar"},
+    }
+)
+```
+
+```py
+response = client.create_transfer_config(
+    parent=project_path,
+    transfer_config={"foo": "bar"},
+)
+```
+
+This call is _invalid_ because it mixes `request` with a keyword argument `transfer_config`.
+Executing this code will result in an error:
+
+```py
+response = client.synthesize_speech(
+    request={"parent": project_path},
+    transfer_config= {"foo": "bar"},
+)
+```
+
+> **NOTE:** The `request` parameter of some methods can also contain a more rich set of
+> options that are otherwise not available as explicit keyword only parameters, thus
+> these _must_ be passed through `request`.
+
+
+## Removed Utility Methods
+
+> **WARNING**: Breaking change
+
+Most utility methods such as `project_path()` have been removed. The paths must
+now be constructed manually:
+
+```py
+project_path = f"project/{PROJECT_ID}"
+```
+
+ The only two that remained are `transfer_config_path()` and `parse_transfer_config_path()`.
+
+
+## Removed `client_config` Parameter
+
+The client cannot be constructed with `client_config` argument anymore, this deprecated
+argument has been removed. If you want to customize retry and timeout settings for a particular
+method, you need to do it upon method invocation by passing the custom `timeout` and
+`retry` arguments, respectively.

docs/UPGRADING.md

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+../UPGRADING.md

docs/index.rst

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     Types <datatransfer_v1/types>
 
 
+Migration Guide
+---------------
+
+See the guide below for instructions on migrating to the 2.x release of this library.
+
+.. toctree::
+    :maxdepth: 2
+
+    UPGRADING
+
+
 Changelog
 ---------