Add a summary of the project to the front page

[s-ludwig]

For someone looking at the project for the first time, it can be a bit confusing to understand the project's scope. A slightly more descriptive addition to the "Asynchronous I/O that doesn’t get in your way" should improve this a lot.

See also: http://forum.rejectedsoftware.com/groups/rejectedsoftware.vibed/thread/23690/