Open dev cheat sheet on GitPod start

When GitPod is started, it shows the default README.md which is missing essential information - how to login, for example. I took me almost an hour to login into dev GitLab (&5531 (comment 613105699)) - time long enough for occasional contributor to bounce away.

The necessary info how to login and how to learn the source with common entrypoints could be directly shown on start.

#1178 (closed) is also related.