How do you add "Rating Field" to a content type?
Do it yourself version:

You also can add a custom field, integers, with min 1 and max 5 values. 
After this you only need to theme the display of the field, in a twig template maybe. 

For achieving the star effect, there are plenty of solutions for that on google (css / js perspective).

One of them is to use field type List (integer) and in options use this values (so there will be select/radios with stars shown, but stored as a integer):

1|★
2|★★
3|★★★
4|★★★★
5|★★★★★