Add readme for upgrade-check "subpackage". (#12506)

The intent here is this content will be visible on the pypi page for
apache-airflow-upgrade-check (once it is published.)

This is far from perfect, but the _first_ step is the hardest :)

(cherry picked from commit 47fc5c4)

airflow/upgrade/README.md

@@ -0,0 +1,82 @@
+<!--
+ Licensed to the Apache Software Foundation (ASF) under one
+ or more contributor license agreements.  See the NOTICE file
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+ regarding copyright ownership.  The ASF licenses this file
+ to you 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
+
+   http://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.
+-->
+
+# Apache Airflow Upgrade Check
+
+[![PyPI version](https://badge.fury.io/py/apache-airflow-upgrade-check.svg)](https://badge.fury.io/py/apache-airflow-upgrade-check)
+[![License](http://img.shields.io/:license-Apache%202-blue.svg)](http://www.apache.org/licenses/LICENSE-2.0.txt)
+[![PyPI - Python Version](https://img.shields.io/pypi/pyversions/apache-airflow-upgrade-check.svg)](https://pypi.org/project/apache-airflow-upgrade-check/)
+[![PyPI - Downloads](https://img.shields.io/pypi/dm/apache-airflow-upgrade-check)](https://pypi.org/project/apache-airflow-upgrade-check/)
+[![Twitter Follow](https://img.shields.io/twitter/follow/ApacheAirflow.svg?style=social&label=Follow)](https://twitter.com/ApacheAirflow)
+[![Slack Status](https://img.shields.io/badge/slack-join_chat-white.svg?logo=slack&style=social)](https://s.apache.org/airflow-slack)
+
+This package aims to easy the upgrade journey from [Apache Airflow](https://airflow.apache.org/) 1.10 to 2.0.
+
+While we have put a lot of effort in to making this upgrade as painless as possible, with many changes
+providing upgrade path (where the old code continues to work and prints out a deprecation warning) there were
+unfortunately some breaking changes where we couldn't provide a compatibility shim.
+
+The recommended upgrade path to get to Airflow 2.0.0 is to first upgrade to the latest release in the 1.10
+series (at the time of writing: 1.10.13) and to then run this script.
+
+```bash
+pip install apache-airflow-upgrade-check
+airflow upgrade_check
+```
+
+This will then print out a number of action items that you should follow before upgrading to 2.0.0 or above.
+
+The exit code of the command will be 0 (success) if no problems are reported, or 1 otherwise.
+
+For example:
+
+```
+============================================= STATUS =============================================
+
+Legacy UI is deprecated by default......................................................SUCCESS
+Users must set a kubernetes.pod_template_file value.....................................FAIL
+Changes in import paths of hooks, operators, sensors and others.........................FAIL
+Remove airflow.AirflowMacroPlugin class.................................................SUCCESS
+Jinja Template Variables cannot be undefined............................................SUCCESS
+Fernet is enabled by default............................................................FAIL
+Logging configuration has been moved to new section.....................................SUCCESS
+Connection.conn_id is not unique........................................................SUCCESS
+GCP service account key deprecation.....................................................SUCCESS
+Users must delete deprecated configs for KubernetesExecutor.............................FAIL
+Changes in import path of remote task handlers..........................................SUCCESS
+Chain between DAG and operator not allowed..............................................SUCCESS
+SendGrid email uses old airflow.contrib module..........................................SUCCESS
+Connection.conn_type is not nullable....................................................SUCCESS
+Found 16 problems.
+
+======================================== RECOMMENDATIONS =========================================
+
+Users must set a kubernetes.pod_template_file value
+---------------------------------------------------
+In Airflow 2.0, KubernetesExecutor Users need to set a pod_template_file as a base
+value for all pods launched by the KubernetesExecutor
+
+
+Problems:
+
+  1.  Please create a pod_template_file by running `airflow generate_pod_template`.
+This will generate a pod using your aiflow.cfg settings
+
+...
+```