Add a migration guide.

Author: ueshin

docs/core-migration-guide.md

@@ -22,6 +22,10 @@ license: |
 * Table of contents
 {:toc}
 
+## Upgrading from Core 3.0 to 3.1
+
+- In Spark 3.0 and below, `SparkContext` can be created in executors. Since Spark 3.1, an exception will be thrown when creating `SparkContext` in executors. If you need to create `SparkContext` in executors, you can allow it by setting the configuration `spark.driver.allowSparkContextInExecutors` in Scala/Java executors or `spark.python.allowSparkContextInExecutors` in PySpark executors.
+
 ## Upgrading from Core 2.4 to 3.0
 
 - The `org.apache.spark.ExecutorPlugin` interface and related configuration has been replaced with