add QA section

README.md

@@ -13,6 +13,13 @@ python server.py <port> <webroot>
 ```
 where ```port``` is a int number of which port to use (default: 80), and ```webroot``` is the directory that containing the images(default: current directory).
 
+## 3. Mostly asked problem
+
+Q1. ```KeyError: webp```
+Ans. We use webp format to encode the image and your system have no libwebp installed. 
+    1. you can change the format to others by modifying the code ```img = img.save(img_stream, format='webp')``` in server.py/HTTPImageServer.handle_image(line 165)
+    2. you can install ```libwebp-dev``` and reinstall pillow package for webp support.
+
 ## 3. Structure.
 The server.py mainly provides two api.
 1. To access filelist under the given directory, please access: