Recommend FULL logging level when using SLF4J (#1146)

Fixes #1142 

Using the logger without setting the correct logging level results in no logging at all.

README.md

@@ -488,6 +488,7 @@ public class Example {
   public static void main(String[] args) {
     GitHub github = Feign.builder()
                      .logger(new Slf4jLogger())
+                     .logLevel(Level.FULL)
                      .target(GitHub.class, "https://api.github.com");
   }
 }

slf4j/README.md

@@ -8,5 +8,6 @@ To use SLF4J with Feign, add both the SLF4J module and an SLF4J binding of your
 ```java
 GitHub github = Feign.builder()
                      .logger(new Slf4jLogger())
+                     .logLevel(Level.FULL)
                      .target(GitHub.class, "https://api.github.com");
 ```