proguard config updates

updated proguard configuration for custom Json Adapters @FromJson/@ToJson cases

README.md

@@ -500,6 +500,9 @@ If you are using ProGuard you might need to add the following options:
 -dontwarn okio.**
 -dontwarn javax.annotation.Nullable
 -dontwarn javax.annotation.ParametersAreNonnullByDefault
+-keepclasseswithmembers class * {
+    @com.squareup.moshi.* <methods>;
+}
 ```