[Feature Request] Add Support for Repository Project Cards

[jcudit]

👋 Please add the ability to create a project card on a project board.

The use case I am looking to address benefits mostly from creating, but other operations would be useful too (list, get, update, delete). I suggest moving a card be out of scope if it adds friction to an implementation.

Here is an example configuration to put a standard card into an instructional column for managed projects:

resource "github_repository_project" "project" {
  name       = "A Repository Project"
  repository = "${github_repository.example.name}"
  body       = "This is a repository project."
}

resource "github_project_column" "instructions" {
  project_id = "${github_repository_project.project.id}"
  name       = "Instructions"
}

resource "github_repository_project_card" "card" {
  note       = "Markdown project board instructions"
  column = "${github_project_column.instructions.name}"
}

/cc https://developer.github.com/v3/projects (docs)
/cc https://developer.github.com/v3/projects/cards (docs)
/cc https://github.com/terraform-providers/terraform-provider-github/pull/115 (prior art)