Update Proguard Rules in README.md

[megshao]

It have to be added the Proguard rules, otherwise it Code shrinked with Proguard could't get the response correctly.

[rharter]

I'm not sure that such a broad rule is needed. What's the specific problem you're having? There are known issues with RxJava and Proguard, could that be it?

I have this in my proguard rules and things are working fine (from my branch):

-keepclassmembers class rx.internal.util.unsafe.*ArrayQueue*Field* {
    long producerIndex;
    long consumerIndex;
}
-keepclassmembers class rx.internal.util.unsafe.BaseLinkedQueueProducerNodeRef {
    rx.internal.util.atomic.LinkedQueueNode producerNode;
}
-keepclassmembers class rx.internal.util.unsafe.BaseLinkedQueueConsumerNodeRef {
    rx.internal.util.atomic.LinkedQueueNode consumerNode;
}
-dontwarn sun.misc.Unsafe