add logging to file example to readme

Readme.md

@@ -53,8 +53,8 @@ setInterval(function(){
 
 #### Windows note
 
- On Windows the environment variable is set using the `set` command. 
- 
+ On Windows the environment variable is set using the `set` command.
+
  ```cmd
  set DEBUG=*,-not_this
  ```
@@ -147,6 +147,16 @@ error('now goes to stdout via console.info');
 log('still goes to stdout, but via console.info now');
 ```
 
+### Save debug output to a file
+
+You can save all debug statements to a file by piping them.
+
+Example:
+
+```bash
+$ DEBUG_FD=3 node your-app.js 3> whatever.log
+```
+
 ## Authors
 
  - TJ Holowaychuk