readme: Link 'Using' wiki page instead of 'Getting started'.

The Windows-specific instruction under 'Quick Start' linked the wiki article on getting started developing Rust itself, but the quick start is just about obtaining a working Rust installation. The actual wiki page with Windows-specific instructions was difficult to find.
auto merge of #17876 : ruud-v-a/rust/patch-1, r=alexcrichton

The Windows-specific instruction under 'Quick Start' linked the wiki article on getting started developing Rust itself, but the quick start is just about obtaining a working Rust installation. The actual wiki page with Windows-specific instructions was difficult to find.

The most important thing to note on Windows is that you need mingw-builds, and it is totally not obvious. The only place where I have seen it mentioned is the wiki page, which was difficult to find before.
Add a memoize! macro and use it throughout rustc
Change memoize!() to be a function instead