added help

samples/c/fback_c.c

 #include <opencv2/video/tracking.hpp>
 #include <opencv2/highgui/highgui.hpp>
+#include <stdio.h>
+void help()
+{
+	printf(
+			"This program demonstrate dense \"Farneback\n optical flow\n"
+			"It read from camera 0, and shows how to use and display dense Franeback optical flow\n"
+			"Call:\n"
+			"./fback_c\n\n");
 
+}
 void drawOptFlowMap(const CvMat* flow, CvMat* cflowmap, int step,
                     double scale, CvScalar color)
 {
@@ -20,6 +29,8 @@ int main(int argc, char** argv)
     CvCapture* capture = cvCreateCameraCapture(0);
     CvMat* prevgray = 0, *gray = 0, *flow = 0, *cflow = 0;
 
+    help();
+
     if( !capture )
         return -1;