android studio - build apk release with proguard

[Costas]

write your rules at :
proguard-rules.pro

at build.gradle (module:app), fix the buildtypes tag :

    buildTypes {
        release {
            debuggable false
            minifyEnabled true
            shrinkResources true
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
           // signingConfig signingConfigs.config (used for preconfigured signing)
        }
        debug {
            //signingConfig signingConfigs.config (used for preconfigured signing)
        }
    }

my APK from 2.7mb went 1.8mb

references :
http://stackoverflow.com/a/35145658
https://gist.github.com/maheshwarLigade/7143ae567fe8ac7c1e20