warn of changed behaviour

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 * Support for [plugins](https://github.com/rollup/rollup/wiki/Plugins) ([#207](https://github.com/rollup/rollup/pulls/207))
 * BREAKING – `options.transform`, `options.load`, `options.resolveId`, `options.resolveExternal` and `options.external` are no longer supported, and should be handled by plugins. [More info](https://github.com/rollup/rollup/wiki/Plugins)
+* BREAKING – the .js extension is only added if it looks like there's no extension, allowing e.g. `import data from 'data.json'` (with the appropriate transformer). For safety, always include the file extension – import `./foo.js`, not `./foo`
 
 ## 0.19.2