Add release note about potential compatibilities to CHANGELOG, closes #12688

Author: josevalim

CHANGELOG.md

@@ -74,6 +74,14 @@ A new function, called `Code.with_diagnostics/2`, has been added so this
 information can be leveraged by editors, allowing them to point to several
 errors at once.
 
+### Potential incompatibilities
+
+As part of this effort, the behaviour where undefined variables were
+transformed into nullary function calls, often leading to confusing error
+reports, has been disabled during project compilation. You can invoke
+`Code.compiler_options(on_undefined_variable: :warn)` at the top of
+your `mix.exs` to bring the old behaviour back.
+
 ## Integration with Erlang/OTP logger
 
 This release provides additional features such as global logger