Document --model option for reindex command

Author: arnaudlimbourg

README.md

@@ -91,6 +91,7 @@ And then replace `algoliasearch.register(YourModel)` with `algoliasearch.registe
 ## Commands
 
 * `python manage.py algolia_reindex`: reindex all the registered models. This command will first send all the record to a temporary index and then moves it.
+    * you can pass ``--model`` parameter to reindex a given model
 * `python manage.py algolia_applysettings`: (re)apply the index settings.
 * `python manage.py algolia_clearindex`: clear the index