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)

api/v1alpha1/runner_types.go

@@ -36,6 +36,9 @@ type RunnerSpec struct {
 	// +optional
 	Labels []string `json:"labels,omitempty"`
 
+	// +optional
+	Group string `json:"group,omitempty"`
+
 	// +optional
 	Containers []corev1.Container `json:"containers,omitempty"`
 	// +optional