text：要生成二维码的文本内容（必须）
size：生成二维码大小，单位像素
示例：?text=https%3A%2F%2Fapi.2xb.cn%2FQRCode&size=250
html中调用：<img src=https://api.2xb.cn/QRCode?text=https%3A%2F%2Fapi.2xb.cn%2FQRCode&size=250">