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)

config/crd/bases/actions.summerwind.dev_runnerdeployments.yaml

@@ -538,6 +538,8 @@ spec:
                           - name
                         type: object
                       type: array
+                    group:
+                      type: string
                     image:
                       type: string
                     imagePullPolicy: