add minify warnings example

README.md

@@ -319,9 +319,20 @@ var result = UglifyJS.minify({
 console.log(result.code);
 ```
 
+To produce warnings:
+```
+var result = UglifyJS.minify("function f(){ var u; return 5; }", {
+    warnings: true
+});
+console.log(result.code);     // function f(){return 5}
+console.log(result.warnings); // [ 'Dropping unused variable u [0:1,18]' ]
+console.log(result.error);    // runtime error, not defined in this case
+```
+
 ## Minify options
 
-- `warnings` (default `false`) — pass `true` to display compressor warnings.
+- `warnings` (default `false`) — pass `true` to return compressor warnings 
+  in `result.warnings`. Use the value `"verbose"` for more detailed warnings.
 
 - `parse` (default `{}`) — pass an object if you wish to specify some
   additional [parse options](#parse-options).