First I have to apologize cause I made a mistake here.
I'm not using Marshmallow, actually. I put android 6+ because Nougat is a brand new android system and doesn't have many users yet. So that plus sign (6+) means Marshmallow and above and it's supposed to include Nougat as well. After I have discovered the solution I googled it to check if that function is available in Marshmallow too and the site Webandother (http://www.webandother.com/how-to/android-marshmallow-startup-apps/) says yes and I trusted them and came here and posted the solution like it was a hidden option in android 6, but NO. That site is wrong. I shouldn't have trusted them.
Sorry again. Now, if you're using Nougat and wanna know how to revoke sub-permissions like 1-Contacts > 1.1-Read contacts > 1.2-Write contacts, read the following:
Android Nougat has an option to do this but it's hidden and I found it by chance. You can acces it in Settings > Memory > Apps started on boot > Click on the app you want to change permissions. In my case, I could revoke whatsapp permission to modify contacts.