Add sudo to setup instructions in README.md

[tlshaheen]

Prevents future installers from running into #23

[mikeosunajr]

I think it is generally considered bad practice to use sudo to install npm packages. Our setup script should not require sudo.

[bolinfest]

I think that something else in your toolchain was, arguably erroneously, installed with sudo if you need sudo to run Nuclide's setup script. As such, we will not take this change.