add readme to creditvalidator

creditCardValidator/README.md

@@ -0,0 +1,37 @@
+## How to use
+First terminal
+> go run credit_card.go 8080
+
+Second terminal
+> curl -X POST http://localhost:8080/ -d '{ "number": "4003600000000014" }' 
+
+received result: {"valid":true}
+
+## What learned
+### Create a http server
+> http.HandleFunc("/", creditCardValidator)
+
+- First parameter "/" is the location of url
+    - If this parmater is /test, then the request address is http://localhost:8080/test 
+- Second parameter is a function that deal the http request.
+
+> http.ListenAndServe(":"+port, nil)
+
+Set the listen address and start the http server
+
+### Http handle function
+> func funcNmae(writer http.ResponseWriter, request *http.Request)
+
+- Get the request body
+   > json.NewDecoder(request.Body).Decode((&cardNumber))
+
+- Check the request type
+    > if request.Method != http.MethodPost {}
+
+- Write response 
+    - Response with error
+    > http.Error(writer, "Invalid request method", http.StatusMethodNotAllowed)
+
+    - Reponse with json data
+    > writer.Header().Set("Content-Type", "application/json")
+	writer.Write(jsonResponse)