Add error message guidelines to PR template

Signed-off-by: Karen Feng <[email protected]>

.github/PULL_REQUEST_TEMPLATE

@@ -34,6 +34,7 @@ Please clarify why the changes are needed. For instance,
 Note that it means *any* user-facing change including all aspects such as the documentation fix.
 If yes, please clarify the previous behavior and the change this PR proposes - provide the console output, description and/or an example to show the behavior difference if possible.
 If possible, please also clarify if this is a user-facing change compared to the released Spark versions or within the unreleased branches such as master.
+If this change involves adding or modifying an error message, please read the guideline first: https://spark.apache.org/error-message-guidelines.html
 If no, write 'No'.
 -->