add Automatic-Module-Name into pf4j.jar (#275)

Thanks!

pf4j/pom.xml

@@ -23,6 +23,17 @@
                     <compilerArgument>-proc:none</compilerArgument>
                 </configuration>
             </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-jar-plugin</artifactId>
+                <configuration>
+                    <archive>
+                        <manifestEntries>
+                            <Automatic-Module-Name>org.pf4j</Automatic-Module-Name>
+                        </manifestEntries>
+                    </archive>
+                </configuration>
+            </plugin>
         </plugins>
     </build>