add documentation

README.md

@@ -786,7 +786,7 @@ You can also use conditional compilation via the programmatic API. With the diff
 property name is `global_defs` and is a compressor property:
 
 ```javascript
-var result = uglifyJS.minify(fs.readFileSync("input.js", "utf8"), {
+var result = UglifyJS.minify(fs.readFileSync("input.js", "utf8"), {
     compress: {
         dead_code: true,
         global_defs: {
@@ -796,6 +796,32 @@ var result = uglifyJS.minify(fs.readFileSync("input.js", "utf8"), {
 });
 ```
 
+To replace an identifier with an arbitrary non-constant expression it is
+necessary to prefix the `global_defs` key with `"@"` to instruct UglifyJS
+to parse the value as an expression:
+```javascript
+UglifyJS.minify("alert('hello');", {
+    compress: {
+        global_defs: {
+            "@alert": "console.log"
+        }
+    }
+}).code;
+// returns: 'console.log("hello");'
+```
+
+Otherwise it would be replaced as string literal:
+```javascript
+UglifyJS.minify("alert('hello');", {
+    compress: {
+        global_defs: {
+            "alert": "console.log"
+        }
+    }
+}).code;
+// returns: '"console.log"("hello");'
+```
+
 ### Using native Uglify AST with `minify()`
 ```javascript
 // example: parse only, produce native Uglify AST