Polish the README file a bit

The `README` should now be a little prettier, both [on GitHub][1] and when looking at [the plain text file][2].

[1]: https://github.com/weiss/ejabberd/tree/ad4b41730e488c75b84894f2ff311f5409f32543#ejabberd-community-edition-by-processone
[2]: https://raw.githubusercontent.com/weiss/ejabberd/ad4b41730e488c75b84894f2ff311f5409f32543/README