Auto merge of #2112 - y86-dev:clarify-custom-rustc-issues, r=RalfJung

Clarified issues when building miri with a custom rustc

I came across these issues (see zulip threads [here](https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/.E2.9C.94.20Changes.20not.20in.20effect) and [here](https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/Missing.20.2Elibrustc.2Estamp), issue [here](rust-lang/rust#90244)) and would like to add this small bit to the docs. Feel free to change the wording.

Author: bors

CONTRIBUTING.md

@@ -206,6 +206,14 @@ rustup toolchain link stage2 build/x86_64-unknown-linux-gnu/stage2
 rustup override set stage2
 ```
 
+Important: You need to delete the Miri cache when you change the stdlib; otherwise the
+old, chached version will be used. On Linux, the cache is located at `~/.cache/miri`; the exact
+location is printed after the library build: "A libstd for Miri is now available in ...".
+
+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>.