added download attr to download links on homepage

same as https://github.com/emberjs/website/pull/1490
according to https://developer.mozilla.org/en-US/docs/Web/HTML/Element/a#attr-download the download attribute tells your browser to download the file, rather than opening in a separate tab/window, which then requires you to save.
Merge pull request #1492 from davidpett/patch-3

added download attr to download links on homepage