Note cloning of obj in setPath & updatePath

In response to https://twitter.com/joshuacc/status/494566618605694976 /cc @joshuacc

I checked [CONTRIBUTING.md](https://github.com/documentcloud/underscore-contrib/blob/master/CONTRIBUTING.md) but I'm confused where to add the docs.

Should they be added [here](https://github.com/documentcloud/underscore-contrib/blob/master/underscore.object.builders.js#L81-85) instead? Where are the examples coming from?