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/integration_test.go

@@ -137,6 +137,7 @@ var _ = Context("Inside of a new namespace", func() {
 							Spec: actionsv1alpha1.RunnerSpec{
 								Repository: "test/valid",
 								Image:      "bar",
+								Group:      "baz",
 								Env: []corev1.EnvVar{
 									{Name: "FOO", Value: "FOOVALUE"},
 								},