Redirect to Discussions to ask questions and get more information

[ThibFrgsGmz]

Context

Since June 16, 2021, the Discussions tab is open on the F Prime GitHub project to have a space on the F Prime GitHub to log in, centralize discussions, find information, create issues directly from a discussion, benefit from the voting system.

Rationale

This tab aims to replace over time the Google Group of the community.
Therefore we should inform our community, in the README.md of the project, to use the Google group instead.

Others

But we can still keep track of the existence of the Google Group in the README.md because there is information/questions not present at the moment in the Discussions tab.