Add details on the require interface

Added a note about how to properly use the require interface via the `mocha` binary. This resolves [the confusion](https://github.com/visionmedia/mocha/issues/1160) that folks have when using the require script in a file that gets run using the `node` binary, not `mocha`.
Merge pull request #1239 from mrjoelkemp/patch-1

Add details on the require interface