DOC: discard `README-v3.md`, or merge into `README.md` or `docs/contributing.rst`

[DimitriPapadopoulos]

Describe the issue linked to the documentation

The README-v3.md file should be discarded or updated and merged into README.md or docs/contributing.rst:

Clone V3 branch

[...]

git checkout --track upstream/v3

Suggested fix for documentation

No response