For example:
$ heroku buildpacks:add --index 1 https://github.com/chrismytton/heroku-buildpack-jq.git
$ heroku buildpacks
=== app-name Buildpack URLs
1. https://github.com/chrismytton/heroku-buildpack-jq.git
2. heroku/python
Perform a deploy to rebuild with multi-buildpacks.
You can confirm jq's installation by running:
$ heroku run which jq
/app/vendor/jq/bin/jq
That confirms it's installed and in the PATH.