Clarifying docs for FlagsForFile client_data

README.md

@@ -872,6 +872,15 @@ FlagsForFile(filename, client_data = {'v:version': 704})
 So the `client_data` parameter is a dictionary mapping Vim expression strings to
 their values at the time of the request.
 
+The correct way to define parameters for your `FlagsForFile` function is like
+this:
+
+```python
+def FlagsForFile(filename, **kwargs):
+```
+
+You can then get to `client_data` like this: `kwargs['client_data']`.
+
 Default: `[]`
 
     let g:ycm_extra_conf_vim_data = []