include the announce-mail description into the rather bleak README

Repeat after me: in a github-centric project, the README *is* your official website. If the announce mail is more helpful than the website itself, you're doing it wrong.

You might want to scrap the "installation instruction" part which is partly redundant with the rest of the file. I think however that this high-level explanation is good for people that like to know what they're doing when they follow your steps.