Document how to add a new example

[carolynvs]

Explain that when you add a new example how (with pictures, it's hard to find!) to give the getporter bot permission to push the example bundle and invocation image to the docker hub registry.

[carolynvs]

Now that we don't use docker hub, the trick is to update the visibility of the example to public. The bot has permission to create the repo, but setting it public is not automatic