add docs/app-payment-paypal.md.

docs/app-payment-paypal.md

+#### 申请开通Paypal
+1. [登录/注册](https://www.paypal.com/c2/signin)
+
+2. 打开[paypal开发者中心](https://developer.paypal.com/developer/applications)
+
+3. 如图，依次选择My Apps & Credentials -> Live(如果创建沙箱环境点击Sandbox) -> Create App
+![](https://partner-dcloud-native.oss-cn-hangzhou.aliyuncs.com/images/uniapp/payment/paypal_develop_center.png)
+
+4. 输入App Name,点击Create App
+![](https://partner-dcloud-native.oss-cn-hangzhou.aliyuncs.com/images/uniapp/payment/paypal_create_app.png)
+
+5. 创建完成后，点击AppName
+![](https://partner-dcloud-native.oss-cn-hangzhou.aliyuncs.com/images/uniapp/payment/paypal_setup_app_info.png)
+
+6. 查看Client ID 以及 Secret
+![](https://partner-dcloud-native.oss-cn-hangzhou.aliyuncs.com/images/uniapp/payment/paypal_get_clientid.png)
+
+7. 添加return URL并保存(必须使用小写字母)
+![](https://partner-dcloud-native.oss-cn-hangzhou.aliyuncs.com/images/uniapp/payment/paypal_add_returnurl.png)
+
+8. 勾选Accept payments 并点击Advanced options选择详情配置(点击保存后生效)
+![](https://partner-dcloud-native.oss-cn-hangzhou.aliyuncs.com/images/uniapp/payment/paypal_accept_payments.png)
+
+9. 勾选Log in with PayPal 并点击Advanced options选择详情配置(点击保存后生效)
+![](https://partner-dcloud-native.oss-cn-hangzhou.aliyuncs.com/images/uniapp/payment/paypal_log_in.png)
+
+------
+
+### 使用Paypal支付
+
+1. 在manifest.json文件“App模块配置”项的“Payment(支付)”下，勾选“paypal支付”项并配置相关参数
+![](https://partner-dcloud-native.oss-cn-hangzhou.aliyuncs.com/images/uniapp/payment/paypal_setup_manifest_info.jpg)
+
+2. 通过服务器生成支付订单并获取orderId(服务器获取订单信息详见[paypal API](https://developer.paypal.com/docs/api/orders/v2/)) 
+
+3. 应用中调用支付功能
+    
+| 参数名称    | 参数说明 | 必须 | 
+|-------------|-------|-----|
+| clientId    | 客户端id(获取方式详见paypal开通文档) | 是  |
+| orderId     | 订单id  | 是 | 
+| environment | 运行环境(sandbox/live) |是 |
+| userAction  | 按钮样式(paynow/continue)  | 否 |
+| currency    | [币种](https://developer.paypal.com/docs/api/reference/currency-codes/) (必须大写)   | 否 |
+
+  * uni-app项目示例
+``` js
+uni.getProvider({
+   service: 'payment',
+   success: function (res) {
+     console.log(res.provider)
+     if (~res.provider.indexOf('paypal')) {
+       uni.requestPayment({
+         "provider": "paypal", 
+         "orderInfo": {
+             "clientId": clientId, //客户端id
+             "orderId": orderId,//订单id
+             "userAction":"continue",//  paynow/continue
+             "currency":"USD",//币种  
+             "environment":"sandbox",//运行环境 sandbox/live
+         },
+         success: function (res) {
+           var rawdata = JSON.parse(res.rawdata);
+           console.log("orderId：" + rawdata.orderId);
+         },
+         fail: function (err) {
+           console.log('fail:' + JSON.stringify(err));
+         }
+       })
+     }
+   }
+});
+```
+  * 5+项目示例
+``` js
+//获取支付渠道
+var paypalSev = null;
+plus.payment.getChannels(function(channels) {
+  for (var i in channels) {
+    var channel = channels[i];
+    if (channel.id === 'paypal') {
+      paypalSev = channel;
+    }
+	}
+}, function(e) {
+	console.log("获取支付渠道失败：" + e.message);
+});
+//发起支付
+var statement = {
+    "clientId": clientId, //客户端id
+    "orderId": orderId,//订单id
+    "userAction":"continue",//  paynow/continue
+    "currency":"USD",//币种  
+    "environment":"sandbox",//运行环境 sandbox/live
+};
+plus.payment.request(this.paypalSev, statement, function(result) {
+    var rawdata = JSON.parse(result.rawdata);
+    console.log("orderId：" + rawdata.orderId);
+}, function(e) {
+    console.log("支付失败：" + e.message);
+});
+```
+
+4. 应用发起支付完成后，返回订单id，服务器捕获或授权订单[详见paypal API](https://developer.paypal.com/docs/api/orders/v2/)
+