adding demo readme

Author: yunlongdong

demo/readme.md

@@ -0,0 +1,8 @@
+# Demos of numpy-cnn
+Here we release some supported demos.
+
+## CRAFT text detector
+Demo for scene text detector with model [CRAFT](https://github.com/clovaai/CRAFT-pytorch).
+Run ```python main.py``` inside folder ```craft_text_detector```.
+The detected result is 
+![](https://raw.githubusercontent.com/Image-Py/cnnumpy/master/demo/craft_text_detector/rst.png)