Consider shipping Proguard rules with jar

[Tapchicoma]

I saw #107 issue, but now Android Gradle plugin supports proguard files from jar as well: https://developer.android.com/studio/build/shrink-code#configuration-files:

JAR libraries: /META-INF/proguard/

[gabrielittner]

As an example, here is how Moshi generates proguard rules dynamically in it's annotation processor https://github.com/square/moshi/blob/master/kotlin/codegen/src/main/java/com/squareup/moshi/kotlin/codegen/api/ProguardRules.kt

[rossbacher]

Looks like a perfectly fine opportunity to contribute to me. ;-)