Add information about Proguard rules

[Wojtek1234]

Hey, you should provide proguard rules in Readme.md, EventBus does not do that on their main page, so there is a confusion after obfuscation wtf is going on, why nothing works

here there are
`-keepattributes Annotation
-keepclassmembers class ** {
@org.greenrobot.eventbus.Subscribe ;
}
-keep enum org.greenrobot.eventbus.ThreadMode { *; }

Only required if you use AsyncExecutor
-keepclassmembers class * extends org.greenrobot.eventbus.util.ThrowableFailureEvent {
(java.lang.Throwable);
}`

[AllanWang]

I agree. And keep in mind you can also add proguard through your own library with consumerProguardFiles 'progress-proguard.txt' under defaultConfig