Update README_fr.md

[rodinux]

Instruction to migrate to paheko

Problem

• Description of why you made this PR

Solution

• And how do you fix that problem

PR Status

• [x ] Code finished and ready to be reviewed/tested
• [x ] The fix/enhancement were manually tested (if applicable)

Automatic tests

Automatic tests can be triggered on https://ci-apps-dev.yunohost.org/ after creating the PR, by commenting "!testme", "!gogogadgetoci" or "By the power of systemd, I invoke The Great App CI to test this Pull Request!". (N.B. : for this to work you need to be a member of the Yunohost-Apps organization)

[rodinux]

Please push this instructions how to migrate to Paheko to advise users if exist.

[rodinux]

please @ericgaspar or @alexAubin or @fflorent or @frju365 , I need a review here...

[alexAubin]

PRs are supposed to target testing, not master x_x