Update readme to make the easiest contribution flow the top level item

We can improve the onboarding process for external contributors to the hubs docs site if we make the readme highlight the non-cloning path for contribution (e.g. "Click "edit" on the page you want to edit, make changes, how to add an image, submit for review as a patch)

Start with the repository README, the file named by this issue. Document the non-cloning contribution flow: use the page's edit action, make changes, add an image, and submit the result for review as a patch. Done means this path is highlighted as the top-level onboarding option.

Written by the indexing model from the issue text.