How to find 'proguard.cfg' file

Question

i'm trying to modify my proguard.cfg and add some lines to keep library classes.

-keepattributes *Annotation*
-keepclassmembers class ** {
    @com.squareup.otto.Subscribe public *;
    @com.squareup.otto.Produce public *;
}

but i can't find the proguard.cfg file as you see in picture below :

Build.Gradle :

buildTypes {
        release {
            minifyEnabled true
            shrinkResources true
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }