message: Add gitlab to the supported services for AppVeyor badges.
  This is undocumented but supported behavior for AppVeyor.
diff: @@ -148,7 +148,8 @@ license-file = "..."
  # Travis CI: `repository` is required. `branch` is optional; default is `master`
  travis-ci = { repository = "...", branch = "master" }
  # Appveyor: `repository` is required. `branch` is optional; default is `master`
  -# `service` is optional; valid values are `github` (default) and `bitbucket`
  +# `service` is optional; valid values are `github` (default), `bitbucket`, and
  +# `gitlab`.
  appveyor = { repository = "...", branch = "master", service = "github" }
  ```
  