Tutorial should mention official nightlies

[huonw]

The getting started section only talks about source builds (and windows binaries) not all the new binaries we have.

[swgillespie]

I modified the getting started section a little bit to include this information and be a little more up to date - feedback welcome!

[steveklabnik]

This should be closed now, yes?

[alexcrichton]

Indeed! http://doc.rust-lang.org/tutorial.html#getting-started