Adds RUNNER_GROUP argument to the runner registration (#157)

* Adds RUNNER_GROUP argument to the runner registration

Adds the ability to register a runner to a predefined runner_group

Resolves #137

* Update README with runner group example

- Updates the README with instructions of how to add the runner to a
  group
- Fix code fencing for shell and yaml blocks in the README
- Use consistent bullet points (dash not asterisk)

controllers/runner_controller.go

@@ -291,6 +291,10 @@ func (r *RunnerReconciler) newPod(ctx context.Context, runner v1alpha1.Runner) (
 			Name:  "RUNNER_LABELS",
 			Value: strings.Join(runner.Spec.Labels, ","),
 		},
+		{
+			Name:  "RUNNER_GROUP",
+			Value: runner.Spec.Group,
+		},
 		{
 			Name:  "RUNNER_TOKEN",
 			Value: token,