Update readme

Author: conradz

Readme.md

@@ -30,7 +30,7 @@ console.log(JSON.stringify(output_obj, null, 2));
 
 `options`:
 
-- `filename` - recommended for debugging.
+- `source` - recommended for debugging.
 - `position` - `true` by default.
 
 ### Errors
@@ -39,8 +39,7 @@ Errors will have `err.position` where `position` is:
 
 - `start` - start line and column numbers
 - `end` - end line and column numbers
-- `filename` - filename if passed to options
-- `source` - source CSS string
+- `source` - `options.source` if passed to options
 
 If you create any errors in plugins such as in [rework](https://github.com/reworkcss/rework), you __must__ set the `position` as well for consistency.