Update readme w/ -config option

Author: jwilder

README.md

@@ -11,7 +11,6 @@ This is can be used to generate config files for:
 
 
 [Docker Log Management With Fluentd](http://jasonwilder.com/blog/2014/03/17/docker-log-management-using-fluentd/)
-
 [Automated Nginx Reverse Proxy for Docker](http://jasonwilder.com/blog/2014/03/25/automated-nginx-reverse-proxy-for-docker/)
 
 ===
@@ -21,13 +20,14 @@ To Run:
  `go get github.com/jwilder/docker-gen`
 
  `docker-gen`
- `Usage: docker-gen [-watch=false] [-notify="restart xyz"] <template> [<dest>]`
+ `Usage: docker-gen [-config file] [-watch=false] [-notify="restart xyz"] <template> [<dest>]`
 
 Options:
 * -watch - runs continuously and monitors docker container events.  When containers are started
 or stopped, the template is regenerated.
 * -notify - runs a command after the template is generated.  Useful for restarting nginx, reloading
 haproxy, etc..
+* -config file - Use the specified config file instead of command-line options.
 
 If no `<dest>` file is specified, the output is send to stdout.  Mainly useful for debugging.