You're organizing a competitive rock-skipping league. 6 coaches and 20 players have signed up. Your job is to randomly and fairly determine the teams, assigning players to coaches. Keep in mind that some teams will have three players and some teams will have four players. Given a Series of coaches and a Series of players, create a Series of random coach-to-player mappings. The resulting Series should have coach names in its index and corresponding player names in its values.