Docs: Mention position option for source map generation

Author: andreypopp

Readme.md

@@ -19,11 +19,15 @@ To get compressed output:
 
 To get a source map:
 
+    var ast = parse('body { font-size: 12px; }', {position: true});
     var result = stringify(ast, {sourcemap: true});
 
     result.code // string with CSS
     result.map // source map
 
+Note that the AST should contain position information (`position` option set to
+`true`).
+
 ## Performance
 
   Formats 15,000 lines of CSS (2mb) in 23ms on my macbook air.