message: Update Appveyor badge docs
  * indicate you can specify the appveyor project id if you want to use that instead
  This PR is part of:
diff: @@ -185,7 +185,8 @@ license-file = "..."
  # Appveyor: `repository` is required. `branch` is optional; default is `master`
  # `service` is optional; valid values are `github` (default), `bitbucket`, and
  -# `gitlab`.
  +# `gitlab`; `id` is optional; you can specify the appveyor project id if you
  +# want to use that instead.
  appveyor = { repository = "...", branch = "master", service = "github" }
  # Circle CI: `repository` is required. `branch` is optiona; default is `master`
  