Update readme

[olsza]

• [x] I provide my work under the project license.
• [x] I grant such license of my work as is required for the purposes of future print editions to Ben Straub and Scott Chacon.

Changes

• Atlas references removed
• removed obsolete link progit.org
• changed to secure link https://

Context

As per the current state of the main project, ie https://github.com/progit/progit2/blob/main/README.asc.

[olsza]

Sorry about the PR - I closed it when I realised I had sent a PR that was not for this project :/