Merge pull request #355 from square/eric.20170925.proguard

Update ProGuard config for CheckReturnValue.

README.md

@@ -518,8 +518,7 @@ ProGuard
 If you are using ProGuard you might need to add the following options:
 ```
 -dontwarn okio.**
--dontwarn javax.annotation.Nullable
--dontwarn javax.annotation.ParametersAreNonnullByDefault
+-dontwarn javax.annotation.**
 -keepclasseswithmembers class * {
     @com.squareup.moshi.* <methods>;
 }