This is a Heroku buildpack for downloading files when you deploy your project. Files are downloaded into $ROOT/downloads. It doesn't do anything else, so to actually compile your app you should use heroku-buildpack-multi to combine it with a real buildpack.
To use this buildpack, you should prepare .buildpacks file that contains this buildpack url and your real buildpack url.
$ ls
.buildpacks
...
$ cat .buildpacks
https://github.com/mikepack/heroku-buildpack-downloader
https://github.com/heroku/heroku-buildpack-ruby
$ heroku create --buildpack https://github.com/ddollar/heroku-buildpack-multi
$ git push heroku master
...
Add URLs to download to .downloads. Remember the UNIX standard is to leave the last line blank.
https://www.google.com/images/srpr/logo1w.png
https://www.google.com/images/srpr/logo2w.png
https://www.google.com/images/srpr/logo3w.png
(blank line)
You can verify your downloads exist by running the following command.
$ heroku run "ls -al downloads"
-rw------- 1 u22102 22102 151 2013-11-21 23:22 logo1w.png
-rw------- 1 u22102 22102 151 2013-11-21 23:22 logo2w.png
-rw------- 1 u22102 22102 151 2013-11-21 23:22 logo3w.png