Document pre/post script behavior definition.

Any script can have pre/post scripts, as *lib/run-script.js* will parse the package.json and run them in the correct order (*line 125*).

This is undocumented, although it is used in at least one prominent repos ([angularjs tutorial](https://github.com/angular/angular-phonecat/blob/master/package.json)).

This documents this feature so that it's behavior can be referenced.