Highlight and enhance changelog note for system-ghc behaviour change

ChangeLog.md

@@ -6,12 +6,17 @@ Release notes:
 
 Major changes:
 
-Behavior changes:
+* Stack will now always use its own GHC installation, even when a suitable GHC
+  installation is available on the PATH. To get the old behaviour, use
+  the `--system-ghc` flag or set `system-ghc: true` in your ~/.stack/config.yaml.
 
-* Use a stack-installed, isolated GHC by default, ignoring any system
-  installations (overridable with `--system-ghc`).
+  NB: Scripts that previously used stack in combination with a system GHC
+  installation should now include a `stack setup` line or use the `--install-ghc`
+  flag.
   [#2221](https://github.com/commercialhaskell/stack/issues/2221)
 
+Behavior changes:
+
 Other enhancements:
 
 Bug fixes: