Rollup merge of #44014 - Mark-Simulacrum:update-readme, r=alexcrichton
Rollup merge of #44014 - Mark-Simulacrum:update-readme, r=alexcrichton

Update rustbuild README

Fixes https://github.com/rust-lang/rust/issues/44007.

r? @alexcrichton

GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust