doc: add short introduction about how to change log level (#1484)

FAQ.md

@@ -160,3 +160,15 @@ https://developer.apple.com/download/more/.
 This is an OS incompatible issue, you could fix by these two steps
 1. `brew edit openresty/brew/openresty`
 1. add `\ -fno-stack-check` in with-luajit-xcflags line.
+
+## How to change the log level?
+
+The default log level for APISIX is `warn`. However You can change the log level to `info` if you want to trace the messages print by `core.log.info`.
+
+Steps:
+
+1. Modify the parameter `error_log_level: "warn"` to `error_log_level: "info"` in conf/config.yaml
+
+2. Restart APISIX
+
+Now you can trace the info level log in logs/error.log.

FAQ_CN.md

@@ -111,3 +111,15 @@ curl -i http://127.0.0.1:9080/apisix/admin/routes/2 -H 'X-API-KEY: edd1c9f034335
 
 更多的 lua-resty-radixtree 匹配操作，可查看操作列表：
 https://github.com/iresty/lua-resty-radixtree#operator-list
+
+## 如何修改日志等级
+
+默认的APISIX日志等级为`warn`，如果需要查看`core.log.info`的打印结果需要将日志等级调整为`info`。
+
+具体步骤：
+
+1、修改conf/config.yaml中的nginx log配置参数`error_log_level: "warn"`为`error_log_level: "info"`。
+
+2、重启APISIX
+
+之后便可以在logs/error.log中查看到info的日志了。