Add back in javadoc plugin (#103)

This was removed in error and now the project cannot be released to maven as it is required.

pom.xml

@@ -114,6 +114,20 @@
             </plugin>
 
             <!-- requirement plugins -->
+            <plugin>
+               <groupId>org.apache.maven.plugins</groupId>
+               <artifactId>maven-javadoc-plugin</artifactId>
+               <version>3.6.0</version>
+               <executions>
+                  <execution>
+                    <id>attach-javadocs</id>
+                    <goals>
+                       <goal>jar</goal>
+                    </goals>
+                  </execution>
+              </executions>
+            </plugin>    
+
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-release-plugin</artifactId>