added source for 'A Guide to Spring Boot in Eclipse'

spring-boot/src/main/java/org/baeldung/boot/DemoApplication.java

@@ -0,0 +1,13 @@
+package org.baeldung.boot;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+@SpringBootApplication
+public class DemoApplication {
+
+	public static void main(String[] args) {
+		System.setProperty("spring.config.name", "demo");
+		SpringApplication.run(DemoApplication.class, args);
+	}
+}