Rollup merge of #82937 - wesleywiser:update_cmake_version_in_readme, r=Mark-Simulacrum
Rollup merge of #82937 - wesleywiser:update_cmake_version_in_readme, r=Mark-Simulacrum

Update README.md to use the correct cmake version number

LLVM requires at least cmake 3.13.4 and cmake is only required to build
LLVM.

https://www.llvm.org/docs/CMake.html

Also closes #42555

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