Add explanation of change to migration note

HeartSaVioR · HeartSaVioR · commit c39b06f4b305 · 2019-09-21T12:47:42.000+09:00
diff --git a/docs/core-migration-guide.md b/docs/core-migration-guide.md
@@ -30,3 +30,4 @@ license: |
 
 - In Spark 3.0, deprecated method `AccumulableInfo.apply` have been removed because creating `AccumulableInfo` is disallowed.
 
+- In Spark 3.0, event log file will be written as UTF-8 encoding, and Spark History Server will replay event log files as UTF-8 encoding. Previously Spark writes event log file as default charset of driver JVM process, so SHS of Spark 2.x is needed to read the old event log files in case of incompatible encoding.

Original file line number	Diff line number	Diff line change
`@@ -30,3 +30,4 @@ license: \|`
`30`	`30`
`31`	`31`	- In Spark 3.0, deprecated method `AccumulableInfo.apply` have been removed because creating `AccumulableInfo` is disallowed.
`32`	`32`
	`33`	`+- In Spark 3.0, event log file will be written as UTF-8 encoding, and Spark History Server will replay event log files as UTF-8 encoding. Previously Spark writes event log file as default charset of driver JVM process, so SHS of Spark 2.x is needed to read the old event log files in case of incompatible encoding.`