list available targets in Makefile help
list available targets in Makefile help

  ```console
  $ make help
  Usage: make <PROVIDER>/<FLOW>

  Available targets:
      facebook/implicit
      google/implicit
      spotify/implicit
      auth0/authorization-code
      auth0/implicit
      auth0/pkce

  Examples:
      make facebook/implicit
      make auth0/pkce
  ```

truqu/elm-oauth2