README should point to x.py instead of ./configure && make

[est31]

In README, you see many occurences of ./configure && make being suggested as default ways to build Rust. Shouldn't they be changed to x.py?

cc @alexcrichton

[alexcrichton]

I'd be fine either way, doesn't matter much to me. We should preserve ./configure && make for familiarity, but for cross-platform instructions it may be good to recommend x.py

[est31]

Fixed by fb2d763