Extended readme to clarify direct dependency on Jdk8Module

This fixes zalando#9

README.md

@@ -41,11 +41,14 @@ In case you're using Jackson, make sure you register the module.
 ```java
 ObjectMapper mapper = new ObjectMapper();
 
+mapper.registerModule(new Jdk8Module());
 mapper.registerModule(new ProblemModule());
 // or
 mapper.findAndRegisterModules();
 ```
 
+**Note**: The `ProblemModule` requires the [`Jdk8Module`](https://github.com/FasterXML/jackson-datatype-jdk8) to work.
+
 ## Creating problems
 
 There are different ways to express problems. Ranging from limited, but easy-to-use to highly flexible and extensible, 
@@ -220,7 +223,8 @@ If you already have an exception class that you want to extend, you should imple
 public final class OutOfStockProblem extends BusinessException implements Exceptional
 ```
 
-The Jackson support module will recognize this inteface and deal with the inherited properties from `Throwable` accordingly. Note: This interface only exists, because `Throwable` is a concrete class, rather than an interface.
+The Jackson support module will recognize this inteface and deal with the inherited properties from `Throwable` 
+accordingly. Note: This interface only exists, because `Throwable` is a concrete class, rather than an interface.
 
 ## Handling problems
 

jackson-datatype-problem/pom.xml

@@ -63,6 +63,12 @@
             <artifactId>jackson-databind</artifactId>
             <version>${jackson.version}</version>
         </dependency>
+        <dependency>
+            <groupId>com.fasterxml.jackson.datatype</groupId>
+            <artifactId>jackson-datatype-jdk8</artifactId>
+            <version>${jackson.version}</version>
+            <!-- We don't work without it. Clients still need to register it though. -->
+        </dependency>
         <dependency>
             <groupId>com.google.guava</groupId>
             <artifactId>guava</artifactId>
@@ -98,12 +104,6 @@
             <version>2.0.0</version>
             <scope>test</scope>
         </dependency>
-        <dependency>
-            <groupId>com.fasterxml.jackson.datatype</groupId>
-            <artifactId>jackson-datatype-jdk8</artifactId>
-            <version>${jackson.version}</version>
-            <scope>test</scope>
-        </dependency>
     </dependencies>
 
     <distributionManagement>

jackson-datatype-problem/src/main/java/org/zalando/problem/ProblemModule.java

@@ -48,7 +48,8 @@ public ProblemModule() {
      * @throws IllegalArgumentException if there are duplicate status codes across all status types
      */
     @SafeVarargs
-    public <E extends Enum & StatusType> ProblemModule(final Class<? extends E>... types) throws IllegalArgumentException {
+    public <E extends Enum & StatusType> ProblemModule(final Class<? extends E>... types)
+            throws IllegalArgumentException {
         super(ProblemModule.class.getSimpleName(), getVersion());
 
         setMixInAnnotation(Exceptional.class, ExceptionalMixin.class);
@@ -61,11 +62,13 @@ public <E extends Enum & StatusType> ProblemModule(final Class<? extends E>... t
 
     @SuppressWarnings("deprecation")
     private static Version getVersion() {
-        return VersionUtil.mavenVersionFor(ProblemModule.class.getClassLoader(), "org.zalando", "jackson-datatype-problem");
+        return VersionUtil.mavenVersionFor(ProblemModule.class.getClassLoader(),
+                "org.zalando", "jackson-datatype-problem");
     }
 
     @SafeVarargs
-    private final <E extends Enum & StatusType> ImmutableMap<Integer, StatusType> buildIndex(final Class<? extends E>... types) {
+    private final <E extends Enum & StatusType> ImmutableMap<Integer, StatusType> buildIndex(
+            final Class<? extends E>... types) {
         final Builder<Integer, StatusType> builder = ImmutableMap.builder();
 
         for (final Class<? extends E> type : types) {