adding usage information

apps/annotation/opencv_annotation.cpp

@@ -152,6 +152,15 @@ void get_annotations(Mat input_image, stringstream* output_stream)
 
 int main( int argc, const char** argv )
 {
+    // If no arguments are given, then supply some information on how this tool works
+    if( argc == 1 ){
+       cout << "Usage: " << argv[0] << endl;
+       cout << "  -images <folder_location> [example - /data/testimages/]" << endl;
+       cout << "  -annotations <ouput_file> [example - /data/annotations.txt]" << endl;
+
+       return -1;
+    }
+
     // Read in the input arguments
     string image_folder;
     string annotations;