Adds a command 'composer' to Terminus which you can use just like 'drush' or 'wp'. The results from the composer command are automatically committed to your git repo.
Note that you can only run composer on dev or multidev environments as composer will almost always need to write to the filesystem.
terminus composer "help" --site=my-site --env=dev
terminus composer "install" --site=my-site --env=dev
For help installing, see Terminus's Wiki
terminus help composer for help.