Specific greeting message to welcome casual contributors

As pointed our by in https://bugs.launchpad.net/ddtp-ubuntu/+bug/376464/comments/13, we need to display a greeting message above each page to properly set expectations for casual translators:
Thanks a lot for contributing to making Ubuntu better. Make sure that your translation is of high quality so that it can be reviewed quickly by one of the Ubuntu translators in your language (depending on your language, this can take some time). Once your suggestion is reviewed, it will go live in Ubuntu at the next repository refresh.