docs(readme): add shell script for cleaning files (#20)

README.md

@@ -100,9 +100,15 @@ $ curl -F "oneshot=@x.txt" "<server_address>"
 ```
 
 #### Cleaning up expired files
+Change `/path/to/rusty` to your rustypaste folder and put this script in the `cron`.
 
 ```sh
-$ find upload/ -maxdepth 2 -type f -iname "*.[0-9]*" -exec rm -v {} \;
+#!/bin/env sh
+now=$(date +%s)
+find /path/to/rusty/ -maxdepth 2 -type f -iname "*.[0-9]*" |
+while read filename; do
+	[ "$(( ${filename##*.} / 1000 - ${now} ))" -lt 0 ] && rm ${filename}
+done
 ```
 
 #### URL shortening