How to Work with the Stan Submodule in rstan Repo?

In this rstan repo, stan repo is configured as a submodule. The first time we clone rstan repo, the source of stan will not be downloaded by default. So we need to execute the following git command. Here is more doc about cloning a project with submodules.

$ git clone --recurse-submodules https://github.com/stan-dev/rstan.git

Later, we can update the submodule if necessary with

$ git submodule update --remote