Add information about quickfixj-codegenerator maven plugin to documentation

"Customizing Message Code Generation" article doesn't have information quickfixj-codegenerator.

Maven is standard de facto tool for building java application. And using of quickfixj-codegenerator is the most convenient way for using own dictionary.

So I suggest to add example of using this plugin for maven. Text may be like this:

If you are using the Maven build system, you can also use quickfixj-codegenerator:

<build>
<plugins>
<plugin>
<groupId>org.quickfixj</groupId>
<artifactId>quickfixj-codegenerator</artifactId>
<version>2.1.1</version>
<executions>
<execution>
<id>fixt11</id>
<goals>
<goal>generate</goal>
</goals>
<configuration>
<dictFile>path/to/your/dictionary/file/FIXT11.xml</dictFile>
<packaging>quickfix.fixt11</packaging>
<fieldPackage>quickfix.field</fieldPackage>
</configuration>
</execution>
<execution>
<id>fix50sp2</id>
<goals>
<goal>generate</goal>
</goals>
<configuration>
<dictFile>path/to/your/dictionary/file/FIX50SP2.modified.xml</dictFile>
<packaging>quickfix.fix50sp2</packaging>
<fieldPackage>quickfix.field</fieldPackage>
</configuration>
</execution>
[...]
</plugin>
[...]
</plugins>
</build>