Add usage examples of selector-labels to documentation

[sarialem]

In order to use label selectors other than app, name and app.kubernetes.io/name, the documentation says:

Besides app Flagger supports name and app.kubernetes.io/name selectors. If you use a different convention you can specify your label with the -selector-labels flag.

But doesn't clearly specify how to use the flag.

Would be great if the documentation has better explanation with examples on using it with either kubectl or helm.