explain that this does not allow any new code

Author: RalfJung

text/0000-constants-in-patterns.md

@@ -9,6 +9,10 @@
 When a constant appears as a pattern, this is syntactic sugar for writing a pattern that corresponds to the constant's value by hand.
 This operation is only allowed when (a) the type of the constant implements `PartialEq`, and (b) the *value* of the constant being matched on has "structural equality", which means that `PartialEq` behaves the same way as that desugared pattern.
 
+This RFC does not allow any new code, compared to what already builds on stable today.
+Its purpose is to explain the rules for constants in patterns in one coherent document,
+and to justify why we will start *rejecting* some code that currently works (see the [breaking changes](#breaking-changes) below).
+
 # Motivation
 [motivation]: #motivation