Readme增加一些详细示例 (#48)

* Update README.md，补充示例

Author: xy-peng

README.md

@@ -57,6 +57,98 @@ HttpClient httpClient = builder.build();
 HttpResponse response = httpClient.execute(...);
 ```
 
+参数说明：
+
++ `merchantId`商户号。
++ `merchantSerialNumber`商户证书的证书序列号，请参考[什么是证书序列号](https://wechatpay-api.gitbook.io/wechatpay-api-v3/chang-jian-wen-ti/zheng-shu-xiang-guan#shen-me-shi-zheng-shu-xu-lie-hao)和[如何查看证书序列号](https://wechatpay-api.gitbook.io/wechatpay-api-v3/chang-jian-wen-ti/zheng-shu-xiang-guan#ru-he-cha-kan-zheng-shu-xu-lie-hao)。
++ `merchantPrivateKey`字符串格式的商户私钥，也就是通过证书工具得到的`apiclient_key.pem`文件中的内容。
++ `wechatpayCertificates`微信支付平台证书的实例列表，用于应答签名的验证。你也可以使用后面章节提到的“自动更新证书功能”。
+
+### 示例：获取平台证书
+
+你可以使用`WechatPayHttpClientBuilder`构造的`HttpClient`发送请求和应答了。
+
+```java
+URIBuilder uriBuilder = new URIBuilder("https://api.mch.weixin.qq.com/v3/certificates");
+HttpGet httpGet = new HttpGet(uriBuilder.build());
+httpGet.addHeader("Accept", "application/json");
+
+CloseableHttpResponse response = httpClient.execute(httpGet);
+
+String bodyAsString = EntityUtils.toString(response.getEntity());
+System.out.println(bodyAsString);
+```
+
+### 示例：JSAPI下单
+
+注：
+
++ 我们使用了 jackson-databind 演示拼装 Json，你也可以使用自己熟悉的 Json 库
++ 请使用你自己的测试商户号、appid 以及对应的 openid
+
+```java
+HttpPost httpPost = new HttpPost("https://api.mch.weixin.qq.com/v3/pay/transactions/jsapi");
+httpPost.addHeader("Accept", "application/json");
+httpPost.addHeader("Content-type","application/json; charset=utf-8");
+
+ByteArrayOutputStream bos = new ByteArrayOutputStream();
+ObjectMapper objectMapper = new ObjectMapper();
+
+ObjectNode rootNode = mapper.createObjectNode();
+rootNode.put("mchid","1900009191")
+        .put("appid", "wxd678efh567hg6787")
+        .put("description", "Image形象店-深圳腾大-QQ公仔")
+        .put("notify_url", "https://www.weixin.qq.com/wxpay/pay.php")
+        .put("out_trade_no", "1217752501201407033233368018");
+rootNode.putObject("amount")
+        .put("total", 1);
+rootNode.putObject("payer")
+        .put("openid", "oUpF8uMuAJO_M2pxb1Q9zNjWeS6o");
+
+mapper.writeValue(bos, rootNode);
+    
+httpPost.setEntity(new StringEntity(bos.toString("UTF-8")));
+CloseableHttpResponse response = httpClient.execute(httpPost);
+
+String bodyAsString = EntityUtils.toString(response.getEntity());
+System.out.println(bodyAsString);
+```
+
+### 示例：查单
+
+```java
+URIBuilder uriBuilder = new URIBuilder("https://api.mch.weixin.qq.com/v3/pay/transactions/id/4200000889202103303311396384?mchid=1230000109");
+HttpGet httpGet = new HttpGet(uriBuilder.build());
+httpGet.addHeader("Accept", "application/json");
+
+CloseableHttpResponse response = httpClient.execute(httpGet);
+
+String bodyAsString = EntityUtils.toString(response.getEntity());
+System.out.println(bodyAsString);
+```
+
+### 示例：关单
+
+```java
+HttpPost httpPost = new HttpPost("https://api.mch.weixin.qq.com/v3/pay/transactions/out-trade-no/1217752501201407033233368018/close");
+httpPost.addHeader("Accept", "application/json");
+httpPost.addHeader("Content-type","application/json; charset=utf-8");
+
+ByteArrayOutputStream bos = new ByteArrayOutputStream();
+ObjectMapper objectMapper = new ObjectMapper();
+
+ObjectNode rootNode = mapper.createObjectNode();
+rootNode.put("mchid","1900009191");
+
+mapper.writeValue(bos, rootNode);
+    
+httpPost.setEntity(new StringEntity(bos.toString("UTF-8")));
+CloseableHttpResponse response = httpClient.execute(httpPost);
+
+String bodyAsString = EntityUtils.toString(response.getEntity());
+System.out.println(bodyAsString);
+```
+
 ## 定制
 
 当默认的本地签名和验签方式不适合你的系统时，你可以通过实现`Signer`或者`Verifier`来定制签名和验签。比如，你的系统把商户私钥集中存储，业务系统需通过远程调用进行签名，你可以这样做。