add docs/app-payment-alipay.md.

docs/app-payment-alipay.md

+1. 登录支付宝开发者中心，创建应用，开通App支付功能，配置密钥
+   详见[支付宝官方文档](https://opendocs.alipay.com/open/204/105297/)
+
+2. 在manifest.json文件“App模块配置”项的“Payment(支付)”下，勾选“支付宝支付”项
+![](https://partner-dcloud-native.oss-cn-hangzhou.aliyuncs.com/images/uniapp/payment/alipay_setup_manifest_info.png)
+
+3. 通过服务器生成支付订单,参考:
+   [生成支付订单示例(PHP)](https://github.com/dcloudio/H5P.Server/tree/master/payment/alipayrsa2)
+   [老版本"移动快捷支付"示例代码(PHP)](https://github.com/dcloudio/H5P.Server/tree/master/payment/alipay)
+   [生成支付订单示例(C#)](http://ask.dcloud.net.cn/article/197)
+
+4. 应用中调用支付功能
+  * uni-app项目示例
+``` js
+uni.getProvider({
+   service: 'payment',
+   success: function (res) {
+     console.log(res.provider)
+     if (~res.provider.indexOf('alipay')) {
+       uni.requestPayment({
+         "provider": "alipay", 
+         "orderInfo": orderInfo, //此处为服务器返回的订单信息字符串
+         success: function (res) {
+           var rawdata = JSON.parse(res.rawdata);
+           console.log("支付成功");
+         },
+         fail: function (err) {
+           console.log('支付失败:' + JSON.stringify(err));
+         }
+       })
+     }
+   }
+});
+```
+  * 5+项目示例
+``` js
+//获取支付渠道
+var alipaySev = null;
+plus.payment.getChannels(function(channels) {
+  for (var i in channels) {
+    var channel = channels[i];
+    if (channel.id === 'alipay') {
+      alipaySev = channel;
+    }
+	}
+}, function(e) {
+	console.log("获取支付渠道失败：" + e.message);
+});
+//发起支付
+var statement = orderInfo; //此处为服务器返回的订单信息字符串
+plus.payment.request(alipaySev, statement, function(result) {
+    var rawdata = JSON.parse(result.rawdata);
+    console.log("支付成功");
+}, function(e) {
+    console.log("支付失败：" + e.message);
+});
+```
+