Add instructions to backport a PR (#9052)

* Add instructions to backport a PR

* Clearer image

* Add period in backport instructions

.backportrc.json

@@ -0,0 +1,7 @@
+{
+  "upstream": "apache/incubator-druid",
+  "branches": [
+    { "name": "0.16.0-incubating", "checked": true },
+    { "name": "0.17.0-incubating", "checked": true }
+  ]
+}

README.md

@@ -97,6 +97,8 @@ For instructions on building Druid from source, see [docs/development/build.md](
 
 Please follow the [community guidelines](https://druid.apache.org/community/) for contributing.
 
+For instructions on setting up IntelliJ [dev/intellij-setup.md](dev/intellij-setup.md)
+
 ### License
 
 [Apache License, Version 2.0](http://www.apache.org/licenses/LICENSE-2.0)

dev/backport.md

@@ -0,0 +1,31 @@
+<!--
+  ~ Licensed to the Apache Software Foundation (ASF) under one
+  ~ or more contributor license agreements.  See the NOTICE file
+  ~ distributed with this work for additional information
+  ~ 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.
+  -->
+
+# Backporting a change
+This document contains some examples of how to backport a change from master to another branch. This is not meant to be
+an exhaustive list of how to backport a change.
+
+## Using [sqren/backport](https://github.com/sqren/backport)
+This is a CLI tool that automates the backport process for you once a change is in master.
+
+To setup, follow the instructions in the [repository](https://github.com/sqren/backport)
+A `.backportrc.json` file is maintained in the root of this repository to keep track of the branches that can be
+backported to.
+
+[![example](backport-fail.gif)]