[doc] Initial import of data using reindex

Update the readme to mention that a reindex is needed after adding the algolia trait to a model class.

Author: bobylito

README.md

@@ -89,6 +89,12 @@ class Contact extends Model
 }
 ```
 
+After setting up your model, you need to manually do the initial import of your data. You can do this by calling `reindex` on your model class. Using our previous example, this would be:
+
+```php
+Contact::reindex();
+```
+
 ### Ranking & Relevance
 
 We provide many ways to configure your index settings to tune the overall relevancy but the most important ones are the **searchable attributes** and the attributes reflecting the **record popularity**. You can configure them with the following code: