add javagc document

src/15-javagc/tests/HelloWorld.java

@@ -0,0 +1,15 @@
+public class HelloWorld {
+    public static void main(String[] args) {
+        // loop and sleep for 1 second
+        while (true) {
+            System.out.println("Hello World!");
+            // create an object and let it go out of scope
+            Object obj = new Object();
+            try {
+                Thread.sleep(1000);
+            } catch (InterruptedException e) {
+                e.printStackTrace();
+            }
+        }
+    }
+}

src/15-javagc/tests/Makefile

@@ -0,0 +1,3 @@
+test:
+	javac HelloWorld.java
+	java HelloWorld