README: add instruction for debug logging (#5)

Signed-off-by: Kay Lin <i@v2bv.net>

Author: RedL0tus

README-zh_CN.md

@@ -53,6 +53,13 @@ OPTIONS:
 trigger --config "<配置文件的路径>"
 ```
 
+或者启用 debug 日志：
+```bash
+TRIGGER_LOG=debug trigger --config "<配置文件的路径>"
+```
+
+此处的 debug 可以换成任意你想要的 log level（有 error、warn、info、debug、trace）。
+
 配置
 ----
 

README.md

@@ -34,6 +34,13 @@ Start the program
 trigger --config "<path to config file>"
 ```
 
+Or, start trigger with debug log enabled:  
+```bash
+TRIGGER_LOG=debug trigger --config "<path to config file>"
+```
+
+`debug` can be replaced with any other logging level you like (including `error`, `warn`, `info`, `debug`, `trace`).
+
 Configuration
 -------------