Mention the need for the msvc toolchain on windows in "Other Installation Methods"

[MalbaCato]

... or link to the page in the rustup book.
rustup-init.exe failed to install the toolchain for somebody (possibly user error). it was really annoying finding the manual installation link from a non-windows machine - had to fake a windows user agent to have it show up on www.rust-lang.org/tools/install.