Added (simpler?) instructions to get started with the plugin!

Hi!

First, thanks so much for the plugin, it solved exactly the problem I was having -- that Disqus does not notify of new comments!
However, I forgot my FTP login, so instead of looking for it, I found that it was possible to simply upload this in a zip file from the Plugins upload page!

Github allows downloading the code as zip file, so from there it was very easy. So I've added those instructions in the readme.

Thanks again for the plugin!