update: use the new README

[noraj]

• use the new readme from https://github.com/traefik/traefik-library-image/blob/master/README.md instead of the old one from the archived repo https://github.com/containous/traefik-library-image/blob/master/README.md
• fixes Docker README traefik/traefik-library-image#13
• completes fix: use the new image repository #2040

[ldez]

Sorry, but we don't agree with these changes.

The file https://github.com/traefik/traefik-library-image/blob/master/README.md is not the "new" readme file.
(It's the opposite and I updated this file)

[ldez]

Please close this PR: my PR #2043 do the expected changes.

[noraj]

Please close this PR: my PR #2043 do the expected changes.

🤔

1. Why having Traefik v2 + Traefik v1 examples ?
2. Please change traefik:v2.0 tags to traefik:v2.5 tags
3. For the documentation rather than giving a 1.7 + 2.5 link, why not just giving https://doc.traefik.io/traefik/ without a version then people can choose the version they want
   
   Else it we will need to update the documentation link at each minor version of each branch.
4. Change the old https://community.containo.us to the new https://community.traefik.io/

[ldez]

1. because we want to have both
2. fixed in my PR
3. fixed in my PR
4. fixed in my PR