Link to sudo integration page from README.md

In my experience with Phusion Passenger, a lot of users get confused by the fact that sudo nukes $PATH. They come to me asking "why does sudo passenger-status give me this weird error?". A lot of users don't even know what $PATH is.

I'm linking to the sudo integration page from the README in the hope of reducing user confusion.
Merge pull request #214 from FooBarWidget/patch-1

Link to sudo integration page from README.md