add warning note

Author: guybedford

doc/api/esm.md

@@ -354,6 +354,10 @@ The conditions supported in Node.js are matched in the following order:
 4. `"default"` - the generic fallback that will always match if no other
    more specific condition is matched first. Can be a CommonJS or ES module
    file.
+   
+> Setting any of the above flagged conditions for a published package is not
+recommended until they are unflagged to avoid breaking changes to packages in
+future.
 
 Using the `"require"` condition it is possible to define a package that will
 have a different exported value for CommonJS and ES modules, which can be a