To add rating in comments app you have to add a field for rating. You can use simple select, radio buttons or similar fieldtype.

At first consider how many "rating-steps" you need. You can use your own system. For this example I tell you about standard 5 Star rating.

You need following: 
a) Comments app:
b) Typo-plugin:
c) SQL plugin for displaying the average rating:

1) Create a field in comments app

2) To display the star rating next to the comments add the comment_rating field into the list&search type "seb_comments"
- click on the button [2] and select PHP string in the Typography column. Click on "+" and set Function: str_repeat, String: 

<i class="fa fa-star"></i>

Typography Target = "value"

3) To display the average rating for the article create a SQL query field, and add following query:
SELECT AVG(comment_rating) FROM #__cck_store_form_seb_comment as t1, #__cck_store_form_yourcontenttype as t2 WHERE t1.parent_id = [pk]
-> mind to replace "#__cck_store_form_yourcontenttype" with the Form&Contenttypes name you are using.