add harmony branch details in README (#1507)

diff --git a/README.md b/README.md
index 490f178..260a685 100644 (file)
--- a/README.md
+++ b/README.md
@@ -10,6 +10,9 @@ There's also an
 [in-browser online demo](http://lisperator.net/uglifyjs/#demo) (for Firefox,
 Chrome and probably Safari).
 
+Note: release versions of `uglify-js` only support ECMAScript 5 (ES5). If you wish to minify
+ES2015+ (ES6+) code then please use the [harmony](#harmony) development branch.
+
 Install
 -------
 
@@ -966,3 +969,20 @@ The `source_map_options` (optional) can contain the following properties:
   [codegen]: http://lisperator.net/uglifyjs/codegen
   [compressor]: http://lisperator.net/uglifyjs/compress
   [parser]: http://lisperator.net/uglifyjs/parser
+
+#### Harmony
+
+If you wish to use the experimental [harmony](https://github.com/mishoo/UglifyJS2/commits/harmony)
+branch to minify ES2015+ (ES6+) code please use the following in your `package.json` file:
+
+```
+"uglify-js": "git+https://github.com/mishoo/UglifyJS2.git#harmony"
+```
+
+or to directly install the experimental harmony version of uglify:
+
+```
+npm install --save-dev uglify-js@github:mishoo/UglifyJS2#harmony
+```
+
+See [#448](https://github.com/mishoo/UglifyJS2/issues/448) for additional details.