add a comment to proguard

app/build.gradle

@@ -3,6 +3,7 @@ apply plugin: 'com.android.application'
 android {
     compileSdkVersion 23
     buildToolsVersion "22.0.1"
+
     defaultConfig {
         applicationId "com.simplemobiletools.calculator"
         minSdkVersion 16

app/proguard-rules.pro

@@ -16,6 +16,7 @@
 #   public *;
 #}
 
+# Butterknife
 -keep class butterknife.** { *; }
 -dontwarn butterknife.internal.**
 -keep class **$$ViewBinder { *; }