Installation documentation could use info on mingw

[catamorphism]

bstrie reports that for Windows users, the mingw situation is not easy to understand: "the mingw site refers me to a sourceforge page which tells me to first download the meta-installer on a different sourceforge page which offers two different installers which are not elaborated upon and upon choosing either installer you are given the option of multiple versions of that installer".

Given the apparent lack of independent docs, we should document how to install mingw if it's a prereq for installing Rust on Windows.

[bstrie]

For the record, Rust requires GCC 4.5 to compile on Windows. I'm not sure which version of MinGW comes with this; the most recent version (and the only one that is easily installed) uses 4.7. Still trying to figure out if there's any nice way to make everything work short of inferring which versions of each file belong together and manually dragging DLLs around.

[brson]

I did some tutorial updates over the weekend, and put in more caveats about building on windows, and more asides urging people to read the 'getting started' wiki page, but the situation is still not good.

[brson]

Various people have tweaked the windows docs in the last year. The tutorial emphatically directs people to the getting started page, which has some detailed information about windows. I don't know if that info is sufficient but enough has changed about the doc situation that I'm going to call this resolved.