provide encoder/decoder in a module that addsTo = Feign.Defaults.class

This clarifies that `Feign.Defaults` contains everything needed except an `Encoder` and a `Decoder`.

In order to complete the graph, you'll want to 
  1. use `Feign.builder()` and specify `.decoder()` and `encoder()`
  2. include or specify a codec module `Feign.create(GitHub.class, "https://api.github.com", new GsonModule())`.
  3. create your own module and add `@Provides` methods for codec.

```java
@dagger.Module(addsTo = Feign.Defaults.class)
static class Dependencies {
  @Provides Decoder defaultDecoder() {
    return // my custom
  }

  @Provides Encoder defaultEncoder() {
    return // my custom
  }
}
```