Add robotstxt.api.php for hook documentation

We have started it for core in D7, but there's nothing that says that module maintainers can't implement it for their modules now as well. If we are ever to have a contrib API site, modules would need to implement their own *.api.php files.

Here's the list of some modules that already implement their own *.api.php:
WYSIWYG API
Administration theme
Signup
ImageCache
Web Services

This standard for documentation was decided fairly recently, but I expect more and more contrib modules to be following this standard as it progresses. I see no reason why we shouldn't help set the standard. :)

Thanks very much hass!