link const -> const_safety

Author: RalfJung

const.md

@@ -6,6 +6,7 @@ On top of what applies to [statics](static.md), they are subject to an additiona
 const CONST: T = EXPR;
 ```
 is supposed to behave as-if `EXPR` was written at every use site of `CONST`.
+To make this work, we need to ensure [const safety](const_safety.md).
 
 Based on this requirement, we allow other constants and [promoteds](promotion.md) to read from constants.
 This is why the value of a `const` is subject to validity checks.