fix: add a public class

MavenReportException: Error while generating Javadoc:
Exit code: 1 -
javadoc: error - No public or protected classes found to document.

media-streaming-spring-boot-autoconfigure/src/main/java/com/jmframework/boot/mediastreamingspringbootautoconfigure/Placeholder.java

@@ -0,0 +1,11 @@
+package com.jmframework.boot.mediastreamingspringbootautoconfigure;
+
+/**
+ * Description: Placeholder, change description here.
+ *
+ * @author 钟俊, email: [email protected]
+ * @date 10/16/2020 5:41 PM
+ **/
+public class Placeholder {
+    public String description = "Placeholder";
+}