added remark to rvprof_init

src/rvprof_core.c

@@ -71,6 +71,8 @@ void rvprof_init(const char* output_file){
 
     // register atexit handler to automate cleanup
     register_atexit_handler();
+
+    fprintf(stderr, "rvprof: profiling active.");
 }
 
 void rvprof_set_program_name(const char* program_name){