support setting logger level

docs/en/install/README.md

@@ -278,6 +278,8 @@ Use environment variables is recommended to avoid conflicts during upgrade.
 
 `HTTP_BASIC_AUTH_PASS`: Http basic authentication password, default to `passw0rd`, please change asap
 
+`LOGGER_LEVEL`: specifies the maximum [level](https://github.com/winstonjs/winston#logging-levels) of messages to the console and log file, default to `info`
+
 ### User Authentication
 
 Routes in `protected_route.js` will be protected using HTTP Basic Authentication.

docs/install/README.md

@@ -284,6 +284,8 @@ gcloud app deploy
 
 `HTTP_BASIC_AUTH_PASS`: Http basic authentication 密码, 默认为 `passw0rd`, 请务必修改
 
+`LOGGER_LEVEL`: 指明输出到 console 和日志文件的日志的最大[等级](https://github.com/winstonjs/winston#logging-levels)，默认 `info`
+
 ### 用户认证
 
 `protected_route.js` 内的路由将启用 HTTP Basic Authentication 认证.