Add a comment to ScriptRunner to make it clear it is an internal utility

We keep receiving enhancement requests for this class, but we actually do not need to improve this class as it's just used to load test SQLs in this repo.
As it's a simple, self-contained, standalone class, users are encouraged to improve their own copy.

Author: harawata

src/main/java/org/apache/ibatis/jdbc/ScriptRunner.java

@@ -28,6 +28,11 @@
 import java.util.regex.Pattern;
 
 /**
+ * This is an internal testing utility.<br>
+ * You are welcome to use this class for your own purposes,<br>
+ * but if there is some feature/enhancement you need for your own usage,<br>
+ * please make and modify your own copy instead of sending us an enhancement request.<br>
+ * 
  * @author Clinton Begin
  */
 public class ScriptRunner {