Add comment to explain how to run inference test

Author: panyx0718

paddle/fluid/inference/tests/book/CMakeLists.txt

@@ -24,6 +24,10 @@ function(inference_test TARGET_NAME)
   endforeach()
 endfunction(inference_test)
 
+####################
+# Inference tests here depend on fluid/tests/book
+# User need to run tests in fluid/tests/book first to generate saved model.
+####################
 # This unittest is buggy!
 #inference_test(fit_a_line)
 inference_test(image_classification ARGS vgg resnet)