Update the main README.txt file

* Update the main `README.txt` file to reflect the move from SVN to Git.
* Apply minor wording (and other) improvements to the `README.txt` file.
* Convert the `README.txt` file to Markdown, so that it's [rendered nicely][1] by GitHub.

[1]: https://github.com/weiss/ejabberd-contrib/tree/update-readme