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

Add installation commands to `x` tool README

Author: Dylan-DPC

src/tools/x/README.md

@@ -1,3 +1,10 @@
 # x
 
 `x` invokes `x.py` from any subdirectory.
+
+To install, run the following commands:
+
+```
+$ cd rust/src/tools/x/
+$ cargo install --path .
+```