README: add another bug Miri found

Author: RalfJung

README.md

@@ -245,6 +245,7 @@ Violations of [Stacked Borrows] found that are likely bugs (but Stacked Borrows
 * [`BTreeMap` creating mutable references that overlap with shared references](https://github.com/rust-lang/rust/pull/58431)
 * [`LinkedList` creating overlapping mutable references](https://github.com/rust-lang/rust/pull/60072)
 * [`Vec::push` invalidating existing references into the vector](https://github.com/rust-lang/rust/issues/60847)
+* [`align_to_mut` violating uniqueness of mutable references](https://github.com/rust-lang/rust/issues/68549)
 
 ## License