update readme

Author: matthiaskrgr

tests/crashes/122909.rs

@@ -1,4 +1,6 @@
 //@ compile-flags: -Zpolymorphize=on -Zinline-mir=yes
+//@ known-bug #12345
+
 
 use std::sync::{Arc, Context, Weak};
 

tests/crashes/README.md

@@ -7,7 +7,7 @@ or makes rustc crash in some other way.
 Accepted exit codes are: 101 (may be expanded later)
 
 When adding crashes from https://github.com/rust-lang/rust/issues, the
-issue number should be noted in the file name (12345.rs should suffice).
-
-If you happen to fix one of the crashes, please move it to `tests/ui`! :) 
+issue number should be noted in the file name (12345.rs should suffice)
+and perhaps also inside the file via `//@ known-bug #4321`
 
+If you happen to fix one of the crashes, please move it to `tests/ui`! :)