Add UPGRADING guide.

Author: plamut

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` client drops support for Python
+versions below 3.6. The client surface itself has not changed, but the 1.x series
+will not be receiving any more feature updates or bug fixes. You are thus
+encouraged to upgrade to the 2.x series.
+
+If you experience issues or have questions, please file an
+[issue](https://github.com/googleapis/python-bigquery/issues).
+
+
+## Supported Python Versions
+
+> **WARNING**: Breaking change
+
+The 2.0.0 release requires Python 3.6+.
+
+
+## Supported BigQuery Storage Clients
+
+The 2.0.0 release requires BigQuery Storage `>= 2.0.0`, which dropped support
+for `v1beta1` and `v1beta2` versions of the BigQuery Storage API. If you want to
+use a BigQuery Storage client, it must be the one supporting the `v1` API version.

docs/UPGRADING.md

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+../UPGRADING.md

docs/index.rst

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   reference
   dbapi
 
+Migration Guide
+---------------
+
+See the guide below for instructions on migrating to the 2.x release of this library.
+
+.. toctree::
+    :maxdepth: 2
+
+    UPGRADING
+
 Changelog
 ---------