Docs/tools: add pipe-to-graphite

We wrote [pipe-to-graphite](https://github.com/iFixit/pipe-to-graphite)
a while ago to make it easy to record the output from the status
commands of gearmand, memcached, varnish, etc.  I don't know why we
never added it to the list of tools, so here it is.
Merge pull request #518 from iFixit/pipe-to-graphite

Docs/tools: add pipe-to-graphite