Challenge 19 : added hello world cpp file

Signed-off-by: Rahul Kumar <rkgkumar072@gmail.com>

Author: iamrahul8

contributors/iamrahul8/helloworld.cpp

@@ -0,0 +1,6 @@
+#include <iostream>
+
+int main() {
+    std::cout << "Hello World!";
+    return 0;
+}