math: add French translation to README
math: add French translation to README

Former-commit-id: c0570d56fe8e69e52d745edd0dc411f7754e61a3 [formerly 6348a41a5472be17453c013d4ab3e8356f2aec7d]
Former-commit-id: eb57b8088414190b5a7f8197cdd9c3e55b2e6d9b
EricLagergren/decimal