Rollup merge of #80280 - pierwill:x-readme, r=Mark-Simulacrum
Rollup merge of #80280 - pierwill:x-readme, r=Mark-Simulacrum

Add installation commands to `x` tool README

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