document minify warnings and add an error example (#1973)

README.md

@@ -319,9 +319,27 @@ var result = UglifyJS.minify({
 console.log(result.code);
 ```
 
+To produce warnings:
+```javascript
+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
+```
+
+An error example:
+```javascript
+var result = UglifyJS.minify({"foo.js" : "if (0) else console.log(1);"});
+console.log(JSON.stringify(result.error));
+// {"message":"Unexpected token: keyword (else)","filename":"foo.js","line":1,"col":7,"pos":7}
+```
+
 ## 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).