add empty sources javadoc, per mccull's advice in #1161 (comment)

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=187334858

extensions/multibindings/pom.xml

@@ -25,6 +25,31 @@
           </archive>
         </configuration>
       </plugin>
+      <plugin>
+        <!--
+         | Re-attach empty sources jar as javadoc to satisfy Maven Central requirements
+        -->
+        <groupId>org.codehaus.mojo</groupId>
+        <artifactId>build-helper-maven-plugin</artifactId>
+        <version>1.8</version>
+        <executions>
+          <execution>
+            <id>reattach-sources-as-javadoc</id>
+            <phase>package</phase>
+            <goals>
+              <goal>attach-artifact</goal>
+            </goals>
+            <configuration>
+              <artifacts>
+                <artifact>
+                  <file>${project.build.directory}/${project.build.finalName}-sources.jar</file>
+                  <classifier>javadoc</classifier>
+                </artifact>
+              </artifacts>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
     </plugins>
   </build>
 </project>