message: [skip ci] add a section for submodule
diff: @@ -25,6 +25,13 @@ This creates a directory gudhi-devel, which you are free to move around or renam
  cd gudhi-devel
  ```
  +## Submodule
  +An interface to Hera for Wasserstein distanceis available on an external git repository.
  +Everytime you checkout master or merge from master, afterwards, you will need to run the command:
  +```bash
  +git submodule update --init
  +```
  +
  ## Configuring a remote for a fork
  ```bash
  git remote add upstream https://github.com/GUDHI/gudhi-devel.git
  