Add this information to README, use with Maven/Gradle

[gonzalezsieira]

Repositories have to be properly configured in order to work with Maven. It would be nice to have this information in the README. New users could easily configure the pom.xml file:

<!-- Configure respositories -->
<repositories>
    <repository>
        <id>rosjava</id>
        <url>https://github.com/rosjava/rosjava_mvn_repo/raw/master/</url>
    </repository>
    <repository>
        <id>jfrog</id>
        <url>http://repo.jfrog.org/artifactory/libs-releases/</url>
    </repository>
</repositories>

<dependencies>
    <!-- ROSJAVA Core-->
    <dependency>
        <groupId>org.ros.rosjava_core</groupId>
        <artifactId>rosjava</artifactId>
        <version>0.3.5</version>
    </dependency>
    <!-- Example: std_msgs -->
    <dependency>
        <groupId>org.ros.rosjava_messages</groupId>
        <artifactId>std_msgs</artifactId>
        <version>0.5.11</version>
    </dependency>
</dependencies>

Also, not sure about the Gradle info, but I understand that the jfrog repository has to be declared too to resolve all dependencies.

Regards,