Fix inconsistency between README and cesium language writer dependency in oreczml-core

The following is displayed as dependency in the Readme:

 <dependency>
      <groupId>com.agi</groupId>
      <artifactId>czml-writer</artifactId>
      <version>3.0.0</version>
 </dependency>

While this is the current implementation in the master branch:

        <dependency>
            <groupId>com.agi</groupId>
            <artifactId>cesiumlanguagewriter</artifactId>
            <version>${oreczml.cesiumwriter.version}</version>
        </dependency>

Even though the readme explains the process to start another project based on Oreczml, Oreczml itself cannot be built if the steps are followed as is.

Cheers, Vincent