message: Clarified issues when building miri with a custom rustc
diff: @@ -206,6 +206,13 @@ rustup toolchain link stage2 build/x86_64-unknown-linux-gnu/stage2
  rustup override set stage2
  ```
  +Important: You need to delete the miri cache (located at `~/.cache/miri` on Linux; the exact location is printed after the library build: "A libstd for Miri is now available in ...") when
  +you change the stdlib, otherwise the old, chached version will be used.
  +
  +Note: `./x.py --stage 2 compiler/rustc` currently errors with `thread 'main'
  +panicked at 'fs::read(stamp) failed with No such file or directory (os error 2)`,
  +you can simply ignore that error; Miri will build anyway.
  +
  For more information about building and configuring a local compiler,
  see <https://rustc-dev-guide.rust-lang.org/building/how-to-build-and-run.html>.
  