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I would recommend a Git submodule.
I am successfully using this approach to include Twitter Bootstrap source code into a Django application on Heroku, and since submodules are widely understood (if not universally liked) other developers should have minimal trouble getting up to speed.
But of course there are other options. This Heroku article suggests four, including the one I recommend:
- Git submodules,
- “protected” Git submodules that require credentials for cloning,
- using a
vendor/
directory, and - private package repositories.
Source:stackexchange.com