moodle-tool_mergeusers - renaming README.txt to README.md

    * this helps to see the MD format of the README file.
    * when uploading a new version on Moodle.org, the site allows you to rename the
      file from README.md to README.txt. That's not a problem ;,-)
Merge pull request #26 from jpahullo/readme

Renaming Readme file