update doc

README.md

@@ -5,7 +5,23 @@
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-JSONに特化したHTTPクライアントです。
+Package jsonhttpc provides JSON specialized HTTP Client.
+
+*   request and response is encoded/decoded as JSON automatically.
+*   bit convenient for repeating requests.
+    *   specify base URL - `WithBaseURL()`
+    *   specify HTTP Client - `WithClient()`
+    *   specify HTTP header - `WithHeader()`
+*   tips to customize requests - `Do()`
+    *   `ContentType() string` on `body` overrides "Content-Type" header.
+        (default is "application/json")
+*   error response is decoded as JSON - `Error`
+    *   Most of JSON properties are put into
+        `Properties map[string]interface{}`
+    *   There are some fields for
+        [RFC7808 Problem Details for HTTP APIs][rfc7808]
+
+(Japanese)
 
 *   リクエストとレスポンスは自動的にJSONエンコード/デコードされます
 *   リクエストを繰り返し行うのに少し便利です
@@ -19,4 +35,6 @@ JSONに特化したHTTPクライアントです。
     *   `Error.Properties` に `map[string]interface{}` で入ります
     *   [RFC7808 Problem Details for HTTP APIs][rfc7808] は少し楽できます
 
+## How to use
+
 [rfc7808]:https://tools.ietf.org/html/rfc7807