add trickle bandwidth shaper

See also:

* http://monkey.org/~marius/trickle/
* http://www.tuxradar.com/content/control-your-bandwidth-trickle
* https://github.com/mariusae/trickle
* http://stackoverflow.com/a/19734174/156060

README.md

@@ -216,6 +216,11 @@ Notes:
 
 - Use `zless`, `zmore`, `zcat`, and `zgrep` to operate on compressed files.
 
+- Use `trickle` to simulate slow bandwidth. Here's how to run firefox, limiting up/down network traffic to 100 KB/s:
+```sh
+      trickle -s -u 100 -d 100 firefox
+```
+
 
 ## System debugging