Add documentation for generating autocompletion script during the build

[remkop]

Based on @bobtiernay-okta 's example in #582.

<plugin>
  <groupId>org.codehaus.mojo</groupId>
  <artifactId>exec-maven-plugin</artifactId>
  <version>${exec-maven-plugin.version}</version>
  <executions>
    <execution>
      <id>generate-autocompletion-script</id>
      <phase>package</phase>
      <goals>
        <goal>java</goal>
      </goals>
      <configuration>
        <mainClass>picocli.AutoComplete</mainClass>
        <arguments>
          <argument>--force</argument><!-- overwrite if exists -->
          <argument>--completionScript</argument>
          <argument>${project.build.directory}/mycommand_completion.sh</argument>
          <argument>com.mycompany.mypackage.MyCommand</argument><!-- replace with your class -->
        </arguments>
      </configuration>
    </execution>
  </executions>
</plugin>